Control of solar insolation via thermochromic light-switching gels

sa~rnww~ and Solar

ELSEVIER

Solar Energy Materials and Solar Cells 36 (1995) 339-347

Control of solar insolation via thermochromic light-switching gels A. Beck a,*, W. K6rner a, H. Scheller a, j. Fricke a, W.J. Platzer b, V. Wittwer b a Physikalisches Institut der Universitiit Wiirzburg, A m Hubland D-97074 Wiirzburg, Germany b Fraunhofer-lnstitut fiir Solare Energiesysteme, Oltmannsstr. 5 D-79100 Freiburg, Germany

Received 25 May 1994

Abstract

The investigated thermochromic gels show a phase separation when a characteristic temperature is surpassed. Particles are formed within a polymer network, which are responsible for a drastically increased light scattering efficiency. Accordingly, a reduction in the directional-hemispherical transmission in the solar spectral range from 8 1 + 2 to (10_+15)% for a 1 mm thick foil-gel-foil system was observed. With the help of differential light scattering measurements and Debye's two-phase media model correlation lengths of a c = (100 + 20) nm and a c = (400 + 50) nm were derived for the scattering entities in the clouded case. From the optical thickness of the specimens the switching temperatures are derived. Additionally performed multiflux calculations for a purely scattering switching system allow to predict the radiation transport under variation of the optical thickness and the correlation length.

I. I n t r o d u c t i o n

T h e d e v e l o p m e n t o f m a t e r i a l s for t h e r m a l i n s u l a t i o n a n d c o n t r o l o f insolation in t r a n s p a r e n t l y i n s u l a t e d ( T I ) wall systems [1] is p r e s e n t l y a m a i n t a s k in the field o f passive solar e n e r g y usage. T r a n s p a r e n t i n s u l a t i o n t e c h n o l o g y allows to m i n i m i z e the e n e r g y c o n s u m p t i o n for h e a t i n g r e s i d e n t i a l buildings. I n special cases t h e r m a l self-sufficiency can b e o b t a i n e d [2]. D u e to t h e a p p l i c a t i o n of highly effective T I m a t e r i a l s , such as silica a e r o g e l s [3], h o n e y c o m b o r c a p i l l a r y s t r u c t u r e s [4] m a d e o f d i f f e r e n t synthetic m a t e r i a l s o r glass, the wall c o n s t r u c t i o n r e q u i r e s a r e l i a b l e

* Corresponding author. 0927-0248/95/$09.50 © 1995 Elsevier Science B.V, All rights reserved SSDI 092 7-0248(94)00171 -N

341)

A. Beck et al. / Solar Ener~' Materials and Solar Cells 36 (1995) 339-347

mechanism to control the solar energy input in order to avoid overheating. Under investigation are mechanically driven reflective roller and venetian blinds, electrochromic layers [5], liquid crystal films [6] [7] and thermochromic gels [8]. The tested mechanical systems are expensive and require maintenance. The electrochromic systems, which regulate the solar energy input via light absorption, are not suitable due to overheating and UV induced damage. More promising are switching systems which control the radiation transport via light scattering. Wellknown are special liquid crystal films and thermochromic gels. The former are actively controlled by an applied electrical field, while the transmission of the latter is passively regulated by its temperature.

2. Specimens The specimens investigated are commercial thermochromic products with the brand name "Cloud-Gel", provided by Suntec Inc., Albuquerque,. They consist of a thermochromic layer sandwiched between two clear plastic films. This layer consists of hydrocarbon copolymers dissolved in water; the polymers are elongated for temperatures below the switching point; above this temperature they coagulate forming microscopic globs larger than the solar wavelengths. By altering the concentration of the components the transition temperature may be set between 10°C and 70 ° C. The layer thickness is about 1 ram. Our samples CG1 and CG2 are similar in appearance but have different transition temperatures.

3. Theory To describe the light scattering properties of the thermochromic gels we used the random two-phase media model developed by Debye [9], which consists of two statistically fluctuating phases with different refractive indices. The intensity scattered by these inhomogeneities is given by [10] 10

I~(Q)= I l+(a,,o)2] 2 K

and

/It(Q)

= l l ( Q ) c°s20"

(1)

A

I i and Ill denote the intensities polarized perpendicularly and parallelly to the scattering plane, respectively. Q = (4~r/A) sin 0 / 2 is the momentum transfer and 0 the scattering angle. A is the wavelength within the medium, a c the correlation length, which is a measure for the average size of the structural inhomogeneities. These inhomogeneities are due to the spatial fluctuations of the refraction index of the two phases. For a particulate medium an average diameter D for equivalent spheres can be derived from a c by [8] D ~- 2 l ~ d a , .

(2)

In the case of non-absorbing media the radiative transport is determined by a c, the scattering coefficient S~c,, and the sample thickness d. Measuring the direc-

A. Beck et al. / Solar Energy Materials and Solar Cells 36 (1995) 339-347

341

tional-directional transmission tdd allows to determine the optical thickness z via Beer's law "r = Sscatd = - I n

tdd.

(3)

To calculate the shading efficiency of a plane-parallel foil-gel-foil system, we used the equation of radiative transport for non-emitting media. The numerical solution of this equation, described in detail in Ref. [11], incorporates also the boundary conditions.

4. Experimental setup The optical quantities which describe the macroscopic light-switching properties of the thermochromic layer are the directional-hemispherical transmission and reflection coefficients tab and rah, respectively, in the solar spectral range (300 n m < A _< 2500 nm). These quantities are determined using an integrating sphere arrangement in combination with a double beam spectrophotometer [8]. To allow radiation transport calculations in order to simulate the switching behaviour of the specimens, the temperature dependent directional-directional transmission tdd as well as the differential scattering cross section are investigated at a fixed wavelength of A = 633 nm. tdd was measured using a linear photodiode array containing 512 photodiodes, each of which has a width of 25 ixm [12]. The advantage of this instrument is, that the unscattered laser beam can be separated from the forward scattered intensity and thus scattering coefficients even in strongly scattering media can be obtained. The differential scattering distribution, from which structural informations about the scattering entities can be derived, was probed using a polar nephelometer [8]. The angular dependent solar directional-hemispherical transmittance for a large piece of the sample CG2 was determined with an integrating sphere of 40 cm diameter and a collimated light source. The whole experimental setup is described in Ref. [13]. The large sample in the clouded case was heated using a hairdryer blowing constantly onto the surface of the sample. As the temperature could be hardly controlled, for the clouded case the sample temperature was estimated from the spectral data to be about (43 + 2)° C.

5. Measurements To determine the switching temperature Ts of the two cloud-gel samples we measured tdd at a wavelength of A = 633 nm. In Fig. 1 the temperature dependent optical thickness z, derived via Eq. (3), is depicted. Data can only be derived for ~- < 18. Above this value the direct laser intensity cannot be separated from the forward scattered intensity. To obtain T s, the increase of z was linearly extrapolated towards ~"= 0. The point of intersection with the abscissa is defined as switching temperature. We obtained Ts = (34.2 _+ 0.5)° C (for CG1) and Ts = (27.3

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A. Beck et al. / Solar Energy Materials and Solar Cells 36 (1995) 339-347

20

t

; /

s 0

,~, : ~ , - ~ , = , ~ , - j

22

26

, , ,

30

34

38

TI°C

Fig, 1, Optical thickness at a wavelength of A = 633 nm for the two thermocbromic specimens (El) (CGI) and ( O ) (CG2) as a function of temperature. Additionally plotted are the extrapolation lines to obtain the switching temperatures 7",.

+_ 0.5) ° C (for CG2). /dh and rdh are measured in the solar spectral region in order to characterize the light switching behaviour. In Figs. 2 and 3 the transmission and reflection data of the two different specimens are depicted. In the transparent state both gel systems show a transmittance of about 90% in the visible and near-infrared region. Above A = 1000 nm absorption peaks due to water and the organic gel compounds occur. Below A = 400 nm the transmission decreases due to absorption of the boundary foils and the organic compounds. The nearly wavelength-independent reflection of rdh = 10% is caused by reflection losses at the two air-foil interfaces. Above T s tdh decreases to a value of about 10% (CG1) and 35% (CG2) within about 5K above Ts. Correspondingly, the directional-hemispherical reflection coefficient rdh increases to 80% (CG1) and 65% (CG2). If the transmission spectra are weighted with the solar terrestical spectrum AM1.5, i.e., the solar

100

~

=o

fdh. dear

I\ r\

OJ

50! c" ._o

CJ

250 500

1000

Fig. 2. Spectral directional-hemispherical cloud-gel 1 in the clear and clouded state.

1500 k/nm

transmission tdh ( - - )

2000

2500

and reflection rdh ( - - - )

data o f

A. Beck et al. / Solar Energy Materials and Solar Cells 36 (1995) 339-347

343

100 o~

tdh, clear

g

g

%

s?o

I oo

Isoo

2000

2soo

hlnm Fig. 3. Spectral directional-hemispherical transmission lab ( - - ) and reflection rdh ( - - - ) data of cloud-gel 2 in the clear and clouded state.

spectrum after penetration of 1.5 atmospheric thicknesses, one gets tdh,sol,clear = (81 + 2)% for CG1 and tdh,soLd~ar= (86 + 2) % for CG2, respectively. If the t e m p e r a t u r e increases above the switching point, the weighted transmis+5 +5 sion coefficients /th,sol,clouded = (10-1) % (CG1) and tdh.so],clo,d~d = ( 3 1 2) % (CG2) are obtained within 5K above T s. Thus, a maximal switching level of A?ah,~o~= (71+_5~) % is realized within a small temperature interval. Higher levels may be achieved at higher temperatures. Fig. 4 shows the solar transmittance tah,~or and reflectance rah,~o~ for the sample CG2 as a function of temperature. Only at about 60°C a saturation level seems to be obtained. The angular dependent solar transmittance of sample CG2 shows a functional dependence in the clear case similar to a glass pane, but is rather constant in the clouded case up to 75 °

I

I

I

I

I

I

I

C: on

"-'

0.8

"-

0.6

-'

•

-o-

-

rdh

td,

C

-~- O.L, 4,-¢--

~

0.2 00

0

,,,

20

I

I

I

I

25

30

35

/+0

I

/+5

I

I

50

55

60

T/°C

Fig. 4. Integrated solar transmittance and reflectance for sample CG2 depending on the sample temperature T.

344

A. Beck et al. / Solar Energy Materials and Solar Cells 36 (1995) 339-347 1 (D

- o--o-o--o

-o~--o~

o__o...o..~

08 =

\q\

06

g:

- - o . - - c[eQr

\

- ~-

\,

ctouded

\

0.4.

r-"

02 0

h

,

I

15

,

L

L

30

45

60

75

90

incidence o.ng[e 8 / °

Fig. 5. Angle-dependent solar transmittance tdh for sample CG2 for the clear and the clouded case.

incidence angle (Fig. 5). T h e h e m i s p h e r i c a l - h e m i s p h e r i c a l t r a n s m i t t a n c e thh,sol was calculated for both cases. For the clear sample thh,~ol,~je,~ = (79 __+4) %, in the clouded case thh,~oi,cloud~d= (15 4- 3) % is obtained, In o r d e r to quantify the structural changes d u r i n g the t r a n s i t i o n from the clear to the clouded state differential light scattering m e a s u r e m e n t are performed. Figs. 6 a n d 7 show the scattering distributions for both gels for two t e m p e r a t u r e s below and o n e above T s. With the use of Eq. (1) correlation lengths of ac,cje~r = (210 -+ 10) n m (CG1) a n d ac,c~ r = (400 + 20) n m (CG2) for the t r a n s p a r e n t state are derived. Above T s the scattered light intensity increases strongly with increasing t e m p e r a ture. Only 0.5°C above T s multiple scattering occurs which smooths the intensity distribution more a n d more isotropic. Thus, the data have to be corrected for

I~(8) 1

DO00

0.5

o

0.2 0.1

,

10

'

,

20

,

30 8/° Fig, 6. Perpendicularly polarized scattering intensity I~ (0) versus scattering angle 0 for CGI (T, = 34.2° C). Parameter is the temperature ([])= 26.3° C, ( × ) = 32.5° C, ( + ) = 36.1° C. The solid lines are fit curves according to Eq. (1). The deviation of the measured and the fitted intensity dependence of [(0) in the clear state is caused by surface scattering.

A. Beck et al. / Solar Energy Materials and Solar Cells 36 (1995) 339-347

t

x

Ix(el

o

x

X x

10-}

345

+

o

×

+

I0-2

o

0

10

20

30

el o

~0

Fig. 7. Perpendicularly polarized scattering intensity 1± (0) versus scattering angle 0 for CG2 (Ts = 27.3° C). Parameter is the temperature ([]) = 24.7° C, ( × ) = 27.0 ° C, ( + ) = 28.4° C. The solid lines are fit curves according to Eq. (l).

..................

100 "-

o

BO

o

60

°t~

o~

o o 000

40

0

. . . . . . . . . . . . . . . . . ...................... 0

20

a _-13- [G2

o --_zz

z-z

z_-_T z z z -

z-_-s_-_--_~_-~;.

[G1

10

100 T Fig. 8. Multiflux calculations of the directional-hemispherical transmission tan depending on the optical thickness 7. Parameter is the correlation length determined for (O) cloud-gel 1 (a c = 100 nm) and ([]) cloud-gel 2 (a c = 400 nm) in the clouded state. The horizontal lines represent the transmission values measured at A = 633 nm ( - - ) including the errors (---).

m u l t i p l e s c a t t e r i n g b e f o r e f i t t i n g t h e m t o E q . (1) w h i c h o n l y h o l d s f o r s i n g l e scattering. To take care of the smearing influence of multiple scattering, we p e r f o r m e d M o n t e C a r l o s i m u l a t i o n s as a f u n c t i o n o f o p t i c a l t h i c k n e s s , u s i n g t r i a l c o r r e l a t i o n l e n g t h s [12] (Fig. 8). W i t h t h i s p r o c e d u r e w e d e r i v e d s i n g l e s c a t t e r i n g c o r r e l a t i o n l e n g t h s i n t h e c l o u d e d c a s e o f ac,ctouded = ( 1 0 0 ___2 0 ) n m a t T = 36.1 ° C f o r C G 1 a n d acxlouded = ( 4 0 0 ___5 0 ) n m a t T = 28.4 ° C f o r C G 2 .

6. Radiation transport calculations In order to explain the influence of the scattering properties on the shading level, w e c a l c u l a t e d tdh a t A = 633 n m d e p e n d i n g o n r a n d a c u s i n g t h e m u l t i f l u x

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A. Beck et al. / Solar Energy Materials and Solar Cells 36 (1995) 339-347

model described in [I1]. In Fig. 6 the theoretical results for two different values of a~, which correspond to the measured quantities, are summarized. To estimate the optical thickness in the clouded state we assume that the particle size remains constant above Ts. The increase of • and thus the further reduction of tdh is then induced exclusively by an increase of the particle number density. In Fig. 6 also the determined transmission tdh are depicted. From comparison of the theoretical dependence tdh(~') and the experimental value for tdh in the clouded state values for the optical thickness in the range of ~-= 100 +_ 20 are obtained for both specimens, indicating a highly effective scattering process. 7. Discussion The forward peaked contributions measured in the clear state (Figs. 4 and 5) are due to micrometer-sized surface imperfections like grooves and scratches on the boundary foils and also due to large inhomogeneities of the polymer network of the gel. The smaller at-values of specimen 1 below Ts give a hint, that the polymer network in this case consists of smaller pore than for CG2. After surpassing the switching temperature light scattering caused by small droplets, which are formed during the phase separation, dominates the scattering properties. Calculating the optimal particle size concerning maximal backscattering, and thus a minimal directional-hemispherical transmission, yields a diameter of D = 400 nm for the solar spectrum [14]. With Eq. (2) values of D = 600 nm and D ~- 2.5 ~m for CG1 and CG2, respectively, are obtained. The droplet size of cloud-gel 1 is thus close to the optimum, while the droplets in the second sample are too large. Because of the fact that both gels have nearly the same optical thickness (see Fig. 6) the scattering distribution and thus the particle diameter is solely responsible for the different switching levels. The nearly optimal size of the scattering entities of specimen CG1 together with the high optical thickness are responsible for the large reduction of the t r a n s m i s s i o n Aldh,sol = ( 7 1 +1 5 ) % . Decisive for the particle size seems to be the fineness of the polymer network: Thus the particle of CG1 formed during the phase separation process are smaller than those formed in CG2. 8. Conclusion and outlook This study shows that thermochromic layers with a sufficiently large switching level to prevent overheating in transparent insulation systems can be produced today. As the know-how on the correlation between structural build-up and light scattering properties is available, one can now also hope to develop light-switching systems, which do not suffer from freezing at low ambient temperatures. Acknowledgements We are indebted to Day Chahroudi, Suntek Inc., USA, for providing thermochromic cloud-gel layers.

A. Beck et al. / Solar Energy Materials and Solar Cells 36 (1995) 339-347

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References [1] A. Goetzberger and V. Wittwer. Proc. Physics 6. Springer-Verlag, Heidelberg, 1986, pp. 84-93. [2] K. Voss, W. Stahl and A. Goetzberger. The self-sufficient solar house Freiburg: A building supplies itself with energy. 3rd European Conf. on Architecture, Florence, Italy, 1993. [3] J. Fricke. Aerogels. Proc. Physics 6. Springer-Verlag, Heidelberg, 1986. [4] W.J. Platzer. Optimization problems of transparently insulated systems. Proc. 2nd Workshop on Transparent Insulation, Freiburg, 1988. [5] C.G. Granquist. Electrochromic coatings for smart windows: A status report. 2nd World Renewable Energy Congress, Vol. 1, 1992, pp. 114-123. [6] H.R. Wilson. Transmission switching using micro-encapsulated liquid crystal films. Proceedings 5th Int. Transparent Insulation Meeting, Freiburg, 1992. [7] W. K6rner, H. Scheller, A. Beck and J. Fricke. PDLC films for control of light transmission, J. Phys. D27 (1994) 2145-2151. [8] A. Beck, T. Hoffmann, W. K6rner and J. Fricke. Thermochromic gels for control of insolation. Solar Energy 50(5) (1993) 407-414. [9] P. Debye, H.R. Anderson and H. Brumberger. Scattering by an inhomogenious solid II the correlation function and its application. J. Appl. Phys. 28 (1957) 679-683. [10] M. Kerker. The Scattering of Light. Academic Press, NY, 1969. [11] A. Beck, W. K/Jrner, T. Hoffmann and J. Fricke, Light scattering investigations of thermochromic gels. Appl. Opt. 37 (1992) 3533-3539. [12] W. K/Srner. Lichtstreuuntersuchungen an thermochromen Gelen. Diploma Thesis, Universit~it Wiirzburg Report E21-0291-2, Germany (1991). [13] W.J. Platzer, P. Apian-Bennewitz and V. Wittwer. Measurement of hemispherical transmittance of structured materials like transparent insulation materials, Conf. Opt. Mat. Techn. Energy Eft. and Sol. Energy Conv. VII, Proc. SPIE 1272, 12.-13. 3.90 The Hague, 1990, pp. 297. [14] A. Beck, W. Kfrner, T. Hoffmann and J. Fricke. Microstructure investigations of thermochromic gels. 7 Int. Sonnenforum, Frankfurt, 1990, pp. 509-514.