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Solar energy absorption by acrylic coatings–I: absorption characteristics
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Building and Environment 39 (2004) 1313 – 1319
www.elsevier.com/locate/buildenv
Solar energy absorption by acrylic coatings–I: absorption characteristics Marcin Wielocha , Agnieszka J. Klemma;∗ , Piotr Klemmb a
School of Built and Natural Environment, Glasgow Caledonian University, Cowcaddens Road Glasgow G4 OBA UK b Technical University of Lodz, Poland Received 6 January 2004; received in revised form 12 February 2004; accepted 8 March 2004
Abstract This paper presents the results of a larger study on application of acrylic plasters as external coatings in the passive heating systems. The e6ciency of the heating system strongly depends on the physical and optical properties of the used materials, especially those used for the 7nishing coatings. Absorption characteristics as a function of colour and microstructural features of the external layer of coatings have been analysed in course of the presented investigation. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Solar absorption; Absorption coe6cient; Colour measurements; Acrylic coatings
1. Introduction An inevitable energetic crisis associated with limited sources of conventional fuel turned the public attention to alternative sources of energy. During the last couple of decades renewable and pollution free energy sources have been successfully utilized in civil housing heating systems in many countries [1,2]. Taking a very broad view solar energy in civil housing may be utilized in two di>erent ways —an active or a passive way. Passive heating systems play an integral part of a building. It is estimated that they can reduce the energy consumption even by 75% when compared to conventionally designed houses with a similar living space. Architectural design of the passive systems depends very much on the location of a building on the Earth. Passively heated buildings in the Northern Hemisphere have glass-made facades and/or most of their windows facing South. In the Southern Hemisphere—they face North [1,3]. Solar radiation passing through the solar-oriented glass (windows or conservatories) is converted into a heat and di>used within the building in the form of a long wave radiation. It is then absorbed by surfaces of materials inside insulated building and re-emitted during nighttime when the temperature inside the ∗ Corresponding author. Tel.: +44-141-331-3544; fax: +44-141331-3696. E-mail address: [email protected] (A.J. Klemm).
0360-1323/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2004.03.001
building decreases. Since the gained heat is not e6ciently re-radiated outside back through the glass, nor it can escape, the energy is entrapped. This phenomenon is called a “greenhouse e>ect” [1,4]. The e6ciency of the system strongly depends on the properties of used materials, especially 7nishing coatings. The surfaces exposed to the direct sunlight are usually high-density materials, such as concrete, brick, or stone. These materials can store the energy during a sunny day and slowly re-radiate it during nighttime, resulting in a very slow temperature fall within the building. Due to a reasonably high heat gain caused by the incident sunrays, the 7nishing layers in such systems should demonstrate speci7c optical abilities such as relatively high absorption and low emittance [3,5]. Physical and optical properties of the materials collecting the solar radiation as well as environmental factors such as ambient air temperature, solar radiation intensity, direction and velocity of wind, overcast, solar radiation angle, rainfall and potential air pollution, signi7cantly a>ect the e6ciency of the solar heating systems [6]. 2. Energy absorption The Sun is a highly energetic system converting within every second 4 million tons of hydrogen into helium and inversely, at the temperature of hundreds million of Celsius degrees, resulting in about 3:86 × 1023 kWh generated per year. Despite the fact that only 1:78 × 1012 kWh=year
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M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319 2500
Irradiance [W/m2nm]
2000
1500
1000
500
0 300
500
700
900
1100
1300
1500
1700
1900
Wavelength [nm]
Fig. 1. The solar radiation spectrum [3].
reaches the surface of the Earth, the amount of solar energy is still 20 000 times more than a total annual world energy production [2]. The amount of energy reaching the Earth greatly varies with the position of the Sun, more precisely with its vertical angle. The largest amount is received at noon when the Sun rays fall at the right angle and the smallest when the Sun rises and sets. The total amount of monthly radiation keeps increasing from spring to summer, and decreasing gradually until wintertime [1,3]. The intensity of solar radiation, may vary signi7cantly. For example in the industrial areas it may reach 600– 700 W=m2 , 800–900 W=m2 in the urban areas and around 1000–1100 W=m2 in the high mountains regions. The wavelength range of the Sun’s radiation contains between 0.2 and 3:0 m and its energy varies with the wavelength. The solar radiation spectrum, presented in Fig. 1, consists of the ultraviolet range 0.115–0:455 m (15.89% of energy is collected in this region), the visible range 0.455–0:75 m (35.80%), and the infrared range 0.75–1 m (48.30%). 47.37% of the solar energy is concentrated in the wavelength range of 0.75–4 m [3]. Solar energy may be utilized in engineering industry either in the form of light or heat. Solar radiation in the form of light may be converted straightforwardly to electricity by means of photoelectric cells. The thermal processes, due to their complexity, might be classi7ed with regard to obtained temperature. Low-temperature heating systems utilize Lat plate collectors to heat water or any other medium converting and transferring the gained heat. High-temperature systems, on the other hand, require sun tracking mirrors always facing Sun. They might convert the sun radiation directly into electricity by thermoelectric generators [6]. Another form of energy collection is applied in a passive system. The energy is gained and transferred by means of no medium. The natural convection, conduction and radiation processes are utilized to transfer collected energy from a place of its collection to other rooms in a building.
The optical properties of any kind of collector are speci7ed by absorptivity, emissivity, reLectivity and transmissivity, so that solar radiation arriving at a surface of material might be absorbed, reLected or transmitted. According to Kirchho>’s law in conditions of thermal equilibrium the absorptance and emittance are equal. For monochromatic radiation, the sum of these features (functions of wavelength), is unity. A() + T () + R() = 1;
(1)
A() = ();
(2)
where A, , T , R are correspondingly materials absorptivity, emissivity, transmissivity, reLectivity and is a wavelength [6]. For materials which are perfectly opaque ( = 0), absorptivity A is related to the reLectivity R by A = 1 − R. The properties of the materials are de7ned by the absorption/reLection and emission factors. The absorption coe6cient [7] can be expressed as 2 d[1 − R]E ()
= 1 2 ; (3) dE () 1 where R is material reLectivity, E is solar irradiance [8] and is wavelength. The ratio of the amount of reLected light to the total amount of incident light is called the surface reLectance and contains its values between 0 and 1. Colour of a surface can be therefore explained in terms of the reLectance for each component. The visible spectrum of the light contains 6 regions which vary with the wavelength. The approximate ranges of the wavelength for each colour are violet 400–450 nm, blue 450–490 nm, green 490–560 nm, yellow 560–590 nm, orange 590–630 nm, red 630–700 nm [9]. A visual sensation of the colour of surface depends upon the character of illuminating light. In order to unify that, the Commission on Illumination (Commisiion Internacionale d’Eclairage CIE), established in 1931 international standards of illumination sources and recommended
M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319
1315
green and dark violet. The chemical composition of samples was similar: aggregates 70% ± 5% (maximum fraction diameter 2 mm), 7llers 25% ± 5%, binder 3–5% (water dispersion of acrylic resin), and additives. The specimens were stored under normal laboratory conditions at an average temperature of +18◦ C (±2◦ C) and relative humidity around 50% (±5%) for the period of 3 months. In the course of research the following experiments have been carried out. 3.1. Absorption characteristics
Fig. 2. The chromaticity diagram.
a colour speci7cation method based on tristimulus values X; Y; Z [9,10]. X =K S() · x() O · R(); Y =K Z =K
S() · y() O · R(); S() · z() O · R();
(4)
where S is a function of wavelength representing relative power distribution of illuminant, R is a spectral reLectance of the sample, x; O y; O zO are CIE colour matching functions. The chromaticity coordinates which describe the perceived colour information are de7ned as x=
X ; X +Y +Z
z=
Z : X +Y +Z
y=
Y ; X +Y +Z (5)
The sum of these coordinates satis7es the following equation:x + y + z = 1. Graphical representation of chromaticity requires a three-dimensional structure, which is however not very clear and practicable. Thus chromaticity is conventionally presented on a two-dimensional x − y diagram. The graphical representation is called the chromaticity diagram (see Fig. 2). Chromaticities of all colours can be plotted here in order to visualize the results [9]. For instance the x; y; z coordinates for the achromaticity point are: x = 13 , y = 13 , z = 13 . 3. The experiment procedure Five sets of samples were prepared in the form of 3 mm thick acrylic plasters of di>erent colours: white, ecru, red,
The light absorption abilities of the examined plasters have been speci7ed with the aid of spectrophotometer Cary 5 (Varian), which allowed to carry out measurements in the wavelength range 175–3300 nm. The double beam spectrophotometer was equipped with two di>erent sources of light and its detectors. Deuterium discharge lamp emitted radiation was within the wavelength range 175–300 nm, while the tungsten lump radiation spectrum was 300–3300 nm. The wavelength range of 175–800 nm was detected by a photomultiplier and the wavelength 800–3300 nm by a lead sulphide detector. Examinations were carried out in the wavelength range from 299 to 2000 nm. As a consequence, the total (scattered and specular) reLectance characteristics of tested samples have been identi7ed and the absorption A has been established from the following relationship A = 1 − R. Where R is the reLectance. 3.2. Microstructure The internal microstructure has been examined with the use of the mercury intrusion porosimetry (MIP) method. The MIP method allows to determine not only the total volume of the open pores, but also the pore size distribution. In principal the method utilizes a capillary phenomenon. The level of liquid in a capillary depends on a surface tension of the liquid as well as on the wetting angle of the liquid in a vessel. A liquid with a wetting angle exceeding 90◦ cannot spontaneously get into a capillary because of surface tension. It is essential therefore to apply pressure, which allows mercury penetration into the pores inside the medium. Pore diameter thus is a function of surface tension, contact angle and applied pressure [11]. 3.3. Surface roughness The contact method has been applied to describe the geometric microstructure of sample surfaces. The cavity description is based on the measurement of the real length of the pro7le on a reference length. A specially designed experimental set of instruments with a mobile detecting blade has been used to determine the cavity. Any change in a
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M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319
4. Results The results of spectrographic measurements present the light absorption capabilities for each sample (Fig. 5). A maximum absorption value obtained during the measurements of the white colour sample was 96% for the wavelength 300 nm. For longer wavelengths, however, the absorption has dropped drastically to the level of 39% (420 nm) and continued falling down to the edge of the ultraviolet - visible range—800 nm. Since then, in the infrared range the absorption curve started slowly raising up. The spectroscopic characteristic of the ecru colour behaved similarly to the previous specimen. The spike in absorption (96%) for the wavelength 300 nm was followed by a rapid fall reaching 75% at 400 nm, 47% at 500 nm and 7nally 33%. Simultaneously, with the wavelength increase the absorption has also increased. Eventually the absorption at the end of the measured wavelength range settled at the level of 57%. The red colour specimen revealed slightly better absorption capabilities in comparison to two previously measured specimens (white, ecru). The 96% absorption at the shortest measured wavelength decreased to 93% at the UV-VIS edge. Absorption curve has then progressively fallen down to the level of 46% at the VIS-INR edge. Continuation of the absorption curve demonstrates gradual incline up to the value of 57% at 1950 nm. The absorption curve obtained for the green colour specimens displays extremely variable pattern with some relative extremes in the wavelength range 300–800 nm. The highest level of the absorption has been reached at the wavelength 300 nm—96% and 92% around 456 nm. Between these two extremes a decrease to the value of 88% has been found. The longer the wavelength the lower the absorption
Fig. 3. Cavity-cross section of a surface.
Fig. 4. The real pro7le of the surface structure.
surface structure was recorded by the set of detectors scanning the surface in a reference system directions (X; Y; Z) [12,13]. The accuracy of reading in a horizontal plane was 0:01 mm, while the vertical displacement was electronically controlled with the accuracy of 0:001 mm. The cavity is usually understood as a ratio of the lateral surface of the niche Ar to the surface of the entrance to the niche Ao (see Fig. 3). In the undertaken experiment the cavity has been obtained as a ratio of real length of the pro7le to the geometrical length of the examined pro7le (see Fig. 4):
Absorption spectrophotograph 95
Violet 85
Green Absorption [%]
75
65
Red
55
45
Ecru 35
25
15 300
White
500
700
900
1100
1300
Wavelength [nm] Fig. 5. Absorption spectrophotograph.
1500
1700
1900
M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319
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Table 1 Chromaticity coordinates tristimulus values
Table 2 The results of the mercury intrusion porosimetry measurements
Specimen
Specimen (colour)
Total intrusion volume (ml/g)
Total pore area (m2 =g)
Medium pore diameter (m)
Speci7c density (g/ml)
Skeleton density (g/ml)
White Red Dark violet
0.0409 0.0432 0.0397
4.236 4.485 4.094
3.8329 3.2239 3.1120
2.2713 2.2291 2.2402
2.5036 2.4666 2.4588
White Ecru Red Green Dark violet
Chromaticity coordinates Y
x
y
67.9119 61.7228 23.121 16.4401 5.7181
0.3155 0.3475 0.4061 0.3289 0.3293
0.3234 0.3629 0.3379 0.4117 0.3181
has been measured until 49% has been reached at 800 nm. From this point the absorption has been gradually increasing to the level of 60% for 2000 nm. A spectrophotograph obtained for the dark violet specimen displays the highest results of light absorption. Its pattern is completely distinct from the others. It is very uni7ed, with values, oscillating around the range of 94–95% throughout the whole measured wavelength range. The highest ability of the energy absorption, de7ned by the absorption factor (Eq. (1)), has been obtained for the dark violet specimen and reached = 0:94. The specimens in the red and green colour have also demonstrated relatively high capabilities of light absorption ( =0:62 and 0.73, respectively), whereas the rest of the examined specimens have had low factors of the light absorption = 0:34 white,
= 0:39 ecru. The analysis of the partial coe6cients, especially those obtained in the visible range of wavelengths revealed that there has been a semi-linear increase from values of =0:28(for white specimen), through =0:35(ecru),
= 0:68(red), = 0:80(green) and = 0:93 for the violet sample. The results of the colour speci7cation in the form of chromaticity coordinates and tristimulus values have been presented in Table 1. As an example the chromaticity diagram-graphical presentation of the chromaticity coordinates (x; y) obtained for the white specimen has been presented in Fig. 6.
In order to assess the e>ect of the surface geometrical microstructure in the considered cases both porosity and roughness measurements have been carried out on the representative samples. The results of the mercury intrusion porosimetry, are presented in Table 2, include the total pore area, medium pore diameter (the value above and below which 50% of pores have larger and smaller diameter, respectively), the speci7c and skeleton densities. The pore size distribution has been displayed on the differential intrusion and cumulative intrusion curves as a function of the pore diameter. The di>erential intrusion curves obtained for all specimens demonstrate similar features of the internal microstructure. Fig. 7 presents di>erential distribution curves for the red, violet and white samples. For all tested specimens majority of pores had their diameters concentrated between 0.8 and 3 m and between 10 and 80 m. Analogies are also noticeable in the analysis of the both speci7c and skeleton densities. The values of the intrusion volume and the total pore area obtained for the red specimen are insigni7cantly higher than for the other samples. The highest medium pore diameter, however, has been identi7ed for the white specimen. The results of surface roughness measurement, contact pro7les, have been presented in the form of graphs. As an example three contact pro7les for the red sample have been shown in Fig. 8. The highest amplitude of the surface
Fig. 6. The chromaticity diagram for the white colour specimen.
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M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319 0.026 0.024
Log intrusion [mL/g]
0.022 0.020
Red
0.018
Dark violet
0.016
White
0.014 0.012 0.010 0.008 0.006 0.004 0.002 0.000 1.000E+03
1.000E+02
1.000E+01
1.000E+00
1.000E-01
1.000E-02
Pore diameter [µm]
Fig. 7. Di>erential intrusion curves.
Contact surface profiles 0.7 0.6 0.5
Surface roughness z [mm]
0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 0
1
2
3
4
5
6
7
8
9
10
11
x [mm]
Fig. 8. Contact surface pro7les obtained for selected specimen.
roughness recorded for the white specimen was 1:09 mm and the average cavity reached 1.099. For the violet sample the following results have been obtained: the amplitude was 0:934 mm and the cavity 1.094. The amplitude of the surface roughness, in the case of the red specimen is 0:901 mm whilst the cavity 1.094. Table 3 presents the average values for three di>erent samples.
Table 3 The results of surface roughness analysis
5. Final remarks
The light absorption increases gradually, simultaneously with the change of the colour from a light white and ecru to the darkest violet. The partial absorption factors obtained for each wavelength range also express a semi-linear relation between the absorption and the colour of the material. A signi7cant increase in the value of the partial absorption
Although absorption of the solar radiation depends upon many factors, the most important appears to be inLuenced by the colour of the material and the surface characteristics.
Specimen
Average roughness (mm)
Variance
Standard deviation
Mean error
White Red Dark violet
0.268 0.254 0.580
0.036 0.041 0.042
0.183 0.203 0.204
0.006 0.006 0.006
M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319
factor in the visible range from the value of 0.35 for the ecru colour up to 0.68 for the red colour specimen, has been accompanied by a “move” of chromaticity coordinates towards another colour range on a chromaticity diagram. Analysis of the microstructure of surface layer revealed only small di>erences. This may suggest that in this investigation porosity and the pore size distribution did not play a considerable part in the solar absorption process. On the other hand, the geometrical microstructure of the specimen surfaces suggests signi7cant e>ect on the absorption capabilities, especially in the infrared part of the spectrum. The average surface roughness obtained for the darkest specimen has been over two times higher, comparing to the rest of tested samples, what might have a signi7cant meaning. It is worth noticing that the absorption characteristics vary not only in values but also in contrast, the absorption curve of the violet specimen is very much uni7ed across the whole tested wavelength. The wider tested palette of colours would give more precise characteristics of absorption and could lead to the development of comprehensive and accurate databases relating absorption with surface colour and its microstructure. References [1] Cowan HJ. Solar energy applications in the design of buildings. Applied Science 1980. [2] Licholaj L. Analysis of the performance of the passive solar heating systems and the prediction of energy e6ciency. Rzeszow Technical University Press, Rzeszow; 2000 [in Polish].
1319
[3] Sayigh AAM, Bahadori Mehdi N. Solar energy application in buildings. New York: Academic Press; 1979. [4] de Shiller S, Eraus JM, Casabianca G. Solar energy in conventional housing: quantifying and optimising direct sunlight. In: Sayigh AAM, editor. Energy and the environment into 1990s. First World Energy Congress, vol. 4. Solar low energy architecture. Oxford: Pergamon; 1990. [5] Passive and low energy architecture in housing. Proceedings of the Fifth International PLEA Conference Decs, Hungary. Plea 1986. UNESCO, 1986. [6] Patton AR. Solar energy for heating and cooling of buildings. Park Ridge, NJ: Noyes Data Corp.; 1975. [7] Crnjak OZ, Klanjsek GM, Lencek A, Benz N. The preparation and testing of spectrally selective paints on di>erent substrates for solar absorbers. Solar Energy 2001;69(6):131–5. [8] Selected ASTM standards for solar energy. Annual book of The American Society for Testing and Materials. Appendix E 490, table 5. Easton, Maryland, 1981. [9] MacAdam DL. Color measurement: theme and variations. New York: Springer; 1981. [10] Hunt RWG. Measuring colour, 2nd ed. Chilchester, UK: E. Horwood; 1991. [11] Klemm AJ. The e>ects of admixtures of the mechanical properties and microstructural features of cementitious composites subjected to freezing and thawing. PhD thesis, Strathclyde University, 1994. [12] Maslowski T, Rozniakowski K, Wojtatowicz T. Examinations of geometric microstructure of capillary and porous materials with contact method. Institute of Physics, Technical University of Lodz, 2001. [13] Wieloch M. Analysis of the physical parameters of thin 7nishing coatings with di>erent spectral characteristics under destructive local climate conditions. Msc thesis. Technical University of Lodz, 2002 [in Polish].
Building and Environment 39 (2004) 1313 – 1319
www.elsevier.com/locate/buildenv
Solar energy absorption by acrylic coatings–I: absorption characteristics Marcin Wielocha , Agnieszka J. Klemma;∗ , Piotr Klemmb a
School of Built and Natural Environment, Glasgow Caledonian University, Cowcaddens Road Glasgow G4 OBA UK b Technical University of Lodz, Poland Received 6 January 2004; received in revised form 12 February 2004; accepted 8 March 2004
Abstract This paper presents the results of a larger study on application of acrylic plasters as external coatings in the passive heating systems. The e6ciency of the heating system strongly depends on the physical and optical properties of the used materials, especially those used for the 7nishing coatings. Absorption characteristics as a function of colour and microstructural features of the external layer of coatings have been analysed in course of the presented investigation. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Solar absorption; Absorption coe6cient; Colour measurements; Acrylic coatings
1. Introduction An inevitable energetic crisis associated with limited sources of conventional fuel turned the public attention to alternative sources of energy. During the last couple of decades renewable and pollution free energy sources have been successfully utilized in civil housing heating systems in many countries [1,2]. Taking a very broad view solar energy in civil housing may be utilized in two di>erent ways —an active or a passive way. Passive heating systems play an integral part of a building. It is estimated that they can reduce the energy consumption even by 75% when compared to conventionally designed houses with a similar living space. Architectural design of the passive systems depends very much on the location of a building on the Earth. Passively heated buildings in the Northern Hemisphere have glass-made facades and/or most of their windows facing South. In the Southern Hemisphere—they face North [1,3]. Solar radiation passing through the solar-oriented glass (windows or conservatories) is converted into a heat and di>used within the building in the form of a long wave radiation. It is then absorbed by surfaces of materials inside insulated building and re-emitted during nighttime when the temperature inside the ∗ Corresponding author. Tel.: +44-141-331-3544; fax: +44-141331-3696. E-mail address: [email protected] (A.J. Klemm).
0360-1323/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2004.03.001
building decreases. Since the gained heat is not e6ciently re-radiated outside back through the glass, nor it can escape, the energy is entrapped. This phenomenon is called a “greenhouse e>ect” [1,4]. The e6ciency of the system strongly depends on the properties of used materials, especially 7nishing coatings. The surfaces exposed to the direct sunlight are usually high-density materials, such as concrete, brick, or stone. These materials can store the energy during a sunny day and slowly re-radiate it during nighttime, resulting in a very slow temperature fall within the building. Due to a reasonably high heat gain caused by the incident sunrays, the 7nishing layers in such systems should demonstrate speci7c optical abilities such as relatively high absorption and low emittance [3,5]. Physical and optical properties of the materials collecting the solar radiation as well as environmental factors such as ambient air temperature, solar radiation intensity, direction and velocity of wind, overcast, solar radiation angle, rainfall and potential air pollution, signi7cantly a>ect the e6ciency of the solar heating systems [6]. 2. Energy absorption The Sun is a highly energetic system converting within every second 4 million tons of hydrogen into helium and inversely, at the temperature of hundreds million of Celsius degrees, resulting in about 3:86 × 1023 kWh generated per year. Despite the fact that only 1:78 × 1012 kWh=year
1314
M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319 2500
Irradiance [W/m2nm]
2000
1500
1000
500
0 300
500
700
900
1100
1300
1500
1700
1900
Wavelength [nm]
Fig. 1. The solar radiation spectrum [3].
reaches the surface of the Earth, the amount of solar energy is still 20 000 times more than a total annual world energy production [2]. The amount of energy reaching the Earth greatly varies with the position of the Sun, more precisely with its vertical angle. The largest amount is received at noon when the Sun rays fall at the right angle and the smallest when the Sun rises and sets. The total amount of monthly radiation keeps increasing from spring to summer, and decreasing gradually until wintertime [1,3]. The intensity of solar radiation, may vary signi7cantly. For example in the industrial areas it may reach 600– 700 W=m2 , 800–900 W=m2 in the urban areas and around 1000–1100 W=m2 in the high mountains regions. The wavelength range of the Sun’s radiation contains between 0.2 and 3:0 m and its energy varies with the wavelength. The solar radiation spectrum, presented in Fig. 1, consists of the ultraviolet range 0.115–0:455 m (15.89% of energy is collected in this region), the visible range 0.455–0:75 m (35.80%), and the infrared range 0.75–1 m (48.30%). 47.37% of the solar energy is concentrated in the wavelength range of 0.75–4 m [3]. Solar energy may be utilized in engineering industry either in the form of light or heat. Solar radiation in the form of light may be converted straightforwardly to electricity by means of photoelectric cells. The thermal processes, due to their complexity, might be classi7ed with regard to obtained temperature. Low-temperature heating systems utilize Lat plate collectors to heat water or any other medium converting and transferring the gained heat. High-temperature systems, on the other hand, require sun tracking mirrors always facing Sun. They might convert the sun radiation directly into electricity by thermoelectric generators [6]. Another form of energy collection is applied in a passive system. The energy is gained and transferred by means of no medium. The natural convection, conduction and radiation processes are utilized to transfer collected energy from a place of its collection to other rooms in a building.
The optical properties of any kind of collector are speci7ed by absorptivity, emissivity, reLectivity and transmissivity, so that solar radiation arriving at a surface of material might be absorbed, reLected or transmitted. According to Kirchho>’s law in conditions of thermal equilibrium the absorptance and emittance are equal. For monochromatic radiation, the sum of these features (functions of wavelength), is unity. A() + T () + R() = 1;
(1)
A() = ();
(2)
where A, , T , R are correspondingly materials absorptivity, emissivity, transmissivity, reLectivity and is a wavelength [6]. For materials which are perfectly opaque ( = 0), absorptivity A is related to the reLectivity R by A = 1 − R. The properties of the materials are de7ned by the absorption/reLection and emission factors. The absorption coe6cient [7] can be expressed as 2 d[1 − R]E ()
= 1 2 ; (3) dE () 1 where R is material reLectivity, E is solar irradiance [8] and is wavelength. The ratio of the amount of reLected light to the total amount of incident light is called the surface reLectance and contains its values between 0 and 1. Colour of a surface can be therefore explained in terms of the reLectance for each component. The visible spectrum of the light contains 6 regions which vary with the wavelength. The approximate ranges of the wavelength for each colour are violet 400–450 nm, blue 450–490 nm, green 490–560 nm, yellow 560–590 nm, orange 590–630 nm, red 630–700 nm [9]. A visual sensation of the colour of surface depends upon the character of illuminating light. In order to unify that, the Commission on Illumination (Commisiion Internacionale d’Eclairage CIE), established in 1931 international standards of illumination sources and recommended
M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319
1315
green and dark violet. The chemical composition of samples was similar: aggregates 70% ± 5% (maximum fraction diameter 2 mm), 7llers 25% ± 5%, binder 3–5% (water dispersion of acrylic resin), and additives. The specimens were stored under normal laboratory conditions at an average temperature of +18◦ C (±2◦ C) and relative humidity around 50% (±5%) for the period of 3 months. In the course of research the following experiments have been carried out. 3.1. Absorption characteristics
Fig. 2. The chromaticity diagram.
a colour speci7cation method based on tristimulus values X; Y; Z [9,10]. X =K S() · x() O · R(); Y =K Z =K
S() · y() O · R(); S() · z() O · R();
(4)
where S is a function of wavelength representing relative power distribution of illuminant, R is a spectral reLectance of the sample, x; O y; O zO are CIE colour matching functions. The chromaticity coordinates which describe the perceived colour information are de7ned as x=
X ; X +Y +Z
z=
Z : X +Y +Z
y=
Y ; X +Y +Z (5)
The sum of these coordinates satis7es the following equation:x + y + z = 1. Graphical representation of chromaticity requires a three-dimensional structure, which is however not very clear and practicable. Thus chromaticity is conventionally presented on a two-dimensional x − y diagram. The graphical representation is called the chromaticity diagram (see Fig. 2). Chromaticities of all colours can be plotted here in order to visualize the results [9]. For instance the x; y; z coordinates for the achromaticity point are: x = 13 , y = 13 , z = 13 . 3. The experiment procedure Five sets of samples were prepared in the form of 3 mm thick acrylic plasters of di>erent colours: white, ecru, red,
The light absorption abilities of the examined plasters have been speci7ed with the aid of spectrophotometer Cary 5 (Varian), which allowed to carry out measurements in the wavelength range 175–3300 nm. The double beam spectrophotometer was equipped with two di>erent sources of light and its detectors. Deuterium discharge lamp emitted radiation was within the wavelength range 175–300 nm, while the tungsten lump radiation spectrum was 300–3300 nm. The wavelength range of 175–800 nm was detected by a photomultiplier and the wavelength 800–3300 nm by a lead sulphide detector. Examinations were carried out in the wavelength range from 299 to 2000 nm. As a consequence, the total (scattered and specular) reLectance characteristics of tested samples have been identi7ed and the absorption A has been established from the following relationship A = 1 − R. Where R is the reLectance. 3.2. Microstructure The internal microstructure has been examined with the use of the mercury intrusion porosimetry (MIP) method. The MIP method allows to determine not only the total volume of the open pores, but also the pore size distribution. In principal the method utilizes a capillary phenomenon. The level of liquid in a capillary depends on a surface tension of the liquid as well as on the wetting angle of the liquid in a vessel. A liquid with a wetting angle exceeding 90◦ cannot spontaneously get into a capillary because of surface tension. It is essential therefore to apply pressure, which allows mercury penetration into the pores inside the medium. Pore diameter thus is a function of surface tension, contact angle and applied pressure [11]. 3.3. Surface roughness The contact method has been applied to describe the geometric microstructure of sample surfaces. The cavity description is based on the measurement of the real length of the pro7le on a reference length. A specially designed experimental set of instruments with a mobile detecting blade has been used to determine the cavity. Any change in a
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M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319
4. Results The results of spectrographic measurements present the light absorption capabilities for each sample (Fig. 5). A maximum absorption value obtained during the measurements of the white colour sample was 96% for the wavelength 300 nm. For longer wavelengths, however, the absorption has dropped drastically to the level of 39% (420 nm) and continued falling down to the edge of the ultraviolet - visible range—800 nm. Since then, in the infrared range the absorption curve started slowly raising up. The spectroscopic characteristic of the ecru colour behaved similarly to the previous specimen. The spike in absorption (96%) for the wavelength 300 nm was followed by a rapid fall reaching 75% at 400 nm, 47% at 500 nm and 7nally 33%. Simultaneously, with the wavelength increase the absorption has also increased. Eventually the absorption at the end of the measured wavelength range settled at the level of 57%. The red colour specimen revealed slightly better absorption capabilities in comparison to two previously measured specimens (white, ecru). The 96% absorption at the shortest measured wavelength decreased to 93% at the UV-VIS edge. Absorption curve has then progressively fallen down to the level of 46% at the VIS-INR edge. Continuation of the absorption curve demonstrates gradual incline up to the value of 57% at 1950 nm. The absorption curve obtained for the green colour specimens displays extremely variable pattern with some relative extremes in the wavelength range 300–800 nm. The highest level of the absorption has been reached at the wavelength 300 nm—96% and 92% around 456 nm. Between these two extremes a decrease to the value of 88% has been found. The longer the wavelength the lower the absorption
Fig. 3. Cavity-cross section of a surface.
Fig. 4. The real pro7le of the surface structure.
surface structure was recorded by the set of detectors scanning the surface in a reference system directions (X; Y; Z) [12,13]. The accuracy of reading in a horizontal plane was 0:01 mm, while the vertical displacement was electronically controlled with the accuracy of 0:001 mm. The cavity is usually understood as a ratio of the lateral surface of the niche Ar to the surface of the entrance to the niche Ao (see Fig. 3). In the undertaken experiment the cavity has been obtained as a ratio of real length of the pro7le to the geometrical length of the examined pro7le (see Fig. 4):
Absorption spectrophotograph 95
Violet 85
Green Absorption [%]
75
65
Red
55
45
Ecru 35
25
15 300
White
500
700
900
1100
1300
Wavelength [nm] Fig. 5. Absorption spectrophotograph.
1500
1700
1900
M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319
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Table 1 Chromaticity coordinates tristimulus values
Table 2 The results of the mercury intrusion porosimetry measurements
Specimen
Specimen (colour)
Total intrusion volume (ml/g)
Total pore area (m2 =g)
Medium pore diameter (m)
Speci7c density (g/ml)
Skeleton density (g/ml)
White Red Dark violet
0.0409 0.0432 0.0397
4.236 4.485 4.094
3.8329 3.2239 3.1120
2.2713 2.2291 2.2402
2.5036 2.4666 2.4588
White Ecru Red Green Dark violet
Chromaticity coordinates Y
x
y
67.9119 61.7228 23.121 16.4401 5.7181
0.3155 0.3475 0.4061 0.3289 0.3293
0.3234 0.3629 0.3379 0.4117 0.3181
has been measured until 49% has been reached at 800 nm. From this point the absorption has been gradually increasing to the level of 60% for 2000 nm. A spectrophotograph obtained for the dark violet specimen displays the highest results of light absorption. Its pattern is completely distinct from the others. It is very uni7ed, with values, oscillating around the range of 94–95% throughout the whole measured wavelength range. The highest ability of the energy absorption, de7ned by the absorption factor (Eq. (1)), has been obtained for the dark violet specimen and reached = 0:94. The specimens in the red and green colour have also demonstrated relatively high capabilities of light absorption ( =0:62 and 0.73, respectively), whereas the rest of the examined specimens have had low factors of the light absorption = 0:34 white,
= 0:39 ecru. The analysis of the partial coe6cients, especially those obtained in the visible range of wavelengths revealed that there has been a semi-linear increase from values of =0:28(for white specimen), through =0:35(ecru),
= 0:68(red), = 0:80(green) and = 0:93 for the violet sample. The results of the colour speci7cation in the form of chromaticity coordinates and tristimulus values have been presented in Table 1. As an example the chromaticity diagram-graphical presentation of the chromaticity coordinates (x; y) obtained for the white specimen has been presented in Fig. 6.
In order to assess the e>ect of the surface geometrical microstructure in the considered cases both porosity and roughness measurements have been carried out on the representative samples. The results of the mercury intrusion porosimetry, are presented in Table 2, include the total pore area, medium pore diameter (the value above and below which 50% of pores have larger and smaller diameter, respectively), the speci7c and skeleton densities. The pore size distribution has been displayed on the differential intrusion and cumulative intrusion curves as a function of the pore diameter. The di>erential intrusion curves obtained for all specimens demonstrate similar features of the internal microstructure. Fig. 7 presents di>erential distribution curves for the red, violet and white samples. For all tested specimens majority of pores had their diameters concentrated between 0.8 and 3 m and between 10 and 80 m. Analogies are also noticeable in the analysis of the both speci7c and skeleton densities. The values of the intrusion volume and the total pore area obtained for the red specimen are insigni7cantly higher than for the other samples. The highest medium pore diameter, however, has been identi7ed for the white specimen. The results of surface roughness measurement, contact pro7les, have been presented in the form of graphs. As an example three contact pro7les for the red sample have been shown in Fig. 8. The highest amplitude of the surface
Fig. 6. The chromaticity diagram for the white colour specimen.
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M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319 0.026 0.024
Log intrusion [mL/g]
0.022 0.020
Red
0.018
Dark violet
0.016
White
0.014 0.012 0.010 0.008 0.006 0.004 0.002 0.000 1.000E+03
1.000E+02
1.000E+01
1.000E+00
1.000E-01
1.000E-02
Pore diameter [µm]
Fig. 7. Di>erential intrusion curves.
Contact surface profiles 0.7 0.6 0.5
Surface roughness z [mm]
0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 0
1
2
3
4
5
6
7
8
9
10
11
x [mm]
Fig. 8. Contact surface pro7les obtained for selected specimen.
roughness recorded for the white specimen was 1:09 mm and the average cavity reached 1.099. For the violet sample the following results have been obtained: the amplitude was 0:934 mm and the cavity 1.094. The amplitude of the surface roughness, in the case of the red specimen is 0:901 mm whilst the cavity 1.094. Table 3 presents the average values for three di>erent samples.
Table 3 The results of surface roughness analysis
5. Final remarks
The light absorption increases gradually, simultaneously with the change of the colour from a light white and ecru to the darkest violet. The partial absorption factors obtained for each wavelength range also express a semi-linear relation between the absorption and the colour of the material. A signi7cant increase in the value of the partial absorption
Although absorption of the solar radiation depends upon many factors, the most important appears to be inLuenced by the colour of the material and the surface characteristics.
Specimen
Average roughness (mm)
Variance
Standard deviation
Mean error
White Red Dark violet
0.268 0.254 0.580
0.036 0.041 0.042
0.183 0.203 0.204
0.006 0.006 0.006
M. Wieloch et al. / Building and Environment 39 (2004) 1313 – 1319
factor in the visible range from the value of 0.35 for the ecru colour up to 0.68 for the red colour specimen, has been accompanied by a “move” of chromaticity coordinates towards another colour range on a chromaticity diagram. Analysis of the microstructure of surface layer revealed only small di>erences. This may suggest that in this investigation porosity and the pore size distribution did not play a considerable part in the solar absorption process. On the other hand, the geometrical microstructure of the specimen surfaces suggests signi7cant e>ect on the absorption capabilities, especially in the infrared part of the spectrum. The average surface roughness obtained for the darkest specimen has been over two times higher, comparing to the rest of tested samples, what might have a signi7cant meaning. It is worth noticing that the absorption characteristics vary not only in values but also in contrast, the absorption curve of the violet specimen is very much uni7ed across the whole tested wavelength. The wider tested palette of colours would give more precise characteristics of absorption and could lead to the development of comprehensive and accurate databases relating absorption with surface colour and its microstructure. References [1] Cowan HJ. Solar energy applications in the design of buildings. Applied Science 1980. [2] Licholaj L. Analysis of the performance of the passive solar heating systems and the prediction of energy e6ciency. Rzeszow Technical University Press, Rzeszow; 2000 [in Polish].
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[3] Sayigh AAM, Bahadori Mehdi N. Solar energy application in buildings. New York: Academic Press; 1979. [4] de Shiller S, Eraus JM, Casabianca G. Solar energy in conventional housing: quantifying and optimising direct sunlight. In: Sayigh AAM, editor. Energy and the environment into 1990s. First World Energy Congress, vol. 4. Solar low energy architecture. Oxford: Pergamon; 1990. [5] Passive and low energy architecture in housing. Proceedings of the Fifth International PLEA Conference Decs, Hungary. Plea 1986. UNESCO, 1986. [6] Patton AR. Solar energy for heating and cooling of buildings. Park Ridge, NJ: Noyes Data Corp.; 1975. [7] Crnjak OZ, Klanjsek GM, Lencek A, Benz N. The preparation and testing of spectrally selective paints on di>erent substrates for solar absorbers. Solar Energy 2001;69(6):131–5. [8] Selected ASTM standards for solar energy. Annual book of The American Society for Testing and Materials. Appendix E 490, table 5. Easton, Maryland, 1981. [9] MacAdam DL. Color measurement: theme and variations. New York: Springer; 1981. [10] Hunt RWG. Measuring colour, 2nd ed. Chilchester, UK: E. Horwood; 1991. [11] Klemm AJ. The e>ects of admixtures of the mechanical properties and microstructural features of cementitious composites subjected to freezing and thawing. PhD thesis, Strathclyde University, 1994. [12] Maslowski T, Rozniakowski K, Wojtatowicz T. Examinations of geometric microstructure of capillary and porous materials with contact method. Institute of Physics, Technical University of Lodz, 2001. [13] Wieloch M. Analysis of the physical parameters of thin 7nishing coatings with di>erent spectral characteristics under destructive local climate conditions. Msc thesis. Technical University of Lodz, 2002 [in Polish].