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EXPERIMENTAL INVESTIGATIONS ON THE PERFORMANCE OF GRANULAR SiO2 AEROGEL
AS A TRANSPARENT INSULATION OF MASS WALLS
E. Boy, M. Munding, V. Wittwer
To cite this version:
E. Boy, M. Munding, V. Wittwer. EXPERIMENTAL INVESTIGATIONS ON THE PERFOR- MANCE OF GRANULAR SiO2 AEROGEL AS A TRANSPARENT INSULATION OF MASS WALLS. Journal de Physique Colloques, 1989, 50 (C4), pp.C4-99-C4-105. �10.1051/jphyscol:1989415�.
�jpa-00229491�
REVUE DE PHYSIQUE APPLIQUEE
Colloque C4, Supplement au n 0 4 , Tome 24, Avril 1989
EXPERIMENTAL INVESTIGATIONS ON THE PERFORMANCE OF GRANULAR SiO, AEROGEL AS A TRANSPARENT INSULATION OF MASS WALLS
E. BOY, M. MUNDING and V. WITTWER*
Fraunhofer-Institut fiir Bauphysik, Bereich Wdrme/Klima (Dir.: Prof. Dr.
-Ing. habil. K.A. Gertis) PO Box 80 04 6 9 , 0-7000 Stuttgart 80, F.R.G.
" ~ r a u n h o f e r - ~ n s t i t u t fiir Solare Energiesysteme (Dir.: Prof. Dr. A.
Goetzberger) Oltmannstr. 22, 0-7800 Freiburg, F.R.G.
R6sum6
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En i n t e r p o s a n t un aerogel granule dans l'espace i n t e r m d d i a i r e des v i t r a g e s i s o l a n t s il e s t p o s s i b l e d'am6liorer l e s q u a l i t 6 s i s o l a n t e s de ces 616ments de v i - trage. L ' a p p l i c a t i o n de t e l s 616ments de v i t r a g e sur de p a r o i s e x t 6 r i e u r e s massives q u i sont capables de stocker de 1'Energie peut r e n f o r c e r l e u r pouvoir i s o l a n t par des gains 6nergEtiques compl6mentaires. En moyenne l o r s de l a p 6 r i o d e de chauffage, l a q u a n t i t 6 de chaleur transmise 5 l P i n t 6 r i e u r par des p a r o i s avec une i s o l a t i o n t r a n s - parente e s t e f f e c t i v e m e n t p l u s grande que l e s p e r t e s de chaleur. Les e f f e t s 6nerg6- t i q u e s e t thermiques d'une t e l l e c o n s t r u c t i o n murale sont examinEs au moyen des 616- ments d'essai o r i e n t 6 s au sud. Sur une p a r o i en b6ton de 0,2 m d'&paisseur, on a mont6 deux f e u i l l e s de v e r r e 6 t a n t remplies d'une couche d'aerogel de 12 mm. La va-l e u r k de c e t t e couche i s o l a n t e transparente e s t 1,l Wl(m2K), son degr6 de transmis- s i o n e s t 0,5. La valeur k de l a c o n s t r u c t i o n e n t i G r e e s t 0'97 W/(m2K). Pendant l a p6- r i o d e de chauffage e n t r e f 6 v r i e r e t mai 1986, on a mesur6 une tempgrature maximale de 44 O C 1 l a s u r f a c e e x t 6 r i e u r e de l a p a r o i en b6ton e t de 30 OC
I
sa s u r f a c e i n t 6 - r i e u r e . Pendant l a p6riode de mesure q u i , avec 1712 d e g r 6 - j o u r s , correspond au c l i m a t tnoyen de 1'Europe c e n t r a l e (resp. 3761 Kd pour l a p6riode de chauffage). l a valeur k e f f e c t i v e de c e t t e c o n s t r u c t i o n de p a r o i s'6lGve1
-0,5 W/(m2K). Malgr6 ce c o e f f i - c i e n t k n 6 g a t i f . l e s p e r t e s de chaleur dOes5
l a t r a n s m i s s i o n ne peuvent B t r e absolu- ment empBch6es par une t e l l e c o n s t r u c t i o n de paroi, En e f f e t il y a des p6riodes avec des s u r p l u s de chaleur mais aussi des p6riodes q u i ne permettent que de simples r 6 - duct i o n s des p e r t e s en transmission. La chaleur exc6dente peut nganmoins p e r m e t t r e d ' 6 g a l i s e r l e s p e r t e s de chaleur dOes $ l a v e n t i l a t i o n . Pour 6 v i t e r des gains de cha- l e u r i n d g s i r a h l e s en 6t6, une p r o t e c t i o n s o l a i r e d o i t B t r e pr6vue. L ' a p p l i c a t i o n de t e l s 616ments aux facades des b i t i m e n t s peut c o n t r i b u e rB
l a r 6 d u c t i o n cons6quence del a consommation d'6nergie de chauffage.
Abstract
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By i n s e r t i n g granulated aerogel i n t o t h e intermediate space o f double g l a z - ings, t h e thermal r e s i s t a n c e o f these g l a z i n g u n i t s c o u l d be s i g n i f i c a n t l y improved.When g l a z i n g elements o f t h i s t y p e a r e a p p l i e d t o mass w a l l s , t h e i r s t r o n g i n s u l a t i n g e f f e c t s w i l l be f u r t h e r increased by a d d i t i o n a l passive s o l a r gains. I n t h e average o f t h e h e a t i n g p e r i o d , t r a n s p a r e n t i n s u l a t e d w a l l s convey more gains from t h e o u t s i d e t o t h e i n s i d e t h a n h e a t i s l o s t . Experimental i n v e s t i g a t i o n s have been performed on south f a c i n g w a l l s w i t h a g l a z i n g u n i t composed o f two 4 mm glass panes and a 12 mm l a y e r o f granulated aerogel sandwiched i n between. T h i s u n i t was mounted on a concrete w a l l o f 0.2 m t h i c k n e s s . The w a l l c o n s t r u c t i o n has an o v e r a l l U-value o f 0.97 W/(m2K). The U- value of t h e g l a z i n g u n i t i s 1.1 ~ i ( m * ~ j a'nd t h e t r a n s m i t t a n c e O.S.'Ouring t h e moni- t o r i n g p e r i o d from February through May 1986 t h e concrete w a l l experienced maximum e x t e r i o r s u r f a c e temperatures o f 44 O C ; f o r t h e e n t i r e measurement p e r i o d , t h e e f f e c - t i v e U-value o f t h i s w a l l c o n s t r u c t i o n amounts t o -0.5 Wl(m2K). T h i s p e r i o d i s charac- t e r i s e d by 1712 degree days ( r e s p e c t i v e l y 3761 Kd f o r t h e h e a t i n g p e r i o d ) and a mean Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989415
i n s o l a t i o n o f 2 . 1 kWhl(m2d) i n c i d e n t on t h e south f a c i n g w a l l which a r e t y p i c a l values f o r t h e C e n t r a l European c l i m a t e . B u t even w i t h negative e f f e c t i v e U-values, transmis- s i o n heat losses cannot be prevented a l t o g e t h e r . There a r e p e r i o d s w i t h very h i g h amounts o f s u r p l u s heat, and t h e r e a r e a l s o times t h a t a l l o w o n l y f o r a r e d u c t i o n o f transmission losses. The occasional p e r i o d s w i t h s u r p l u s heat may nevertheless compen- s a t e f o r v e n t i l a t i o n heat losses. To a v o i d overheating i n summer, some e f f i c i e n t s o l a r p r o t e c t i o n must be provided. By u s i n g these g l a z i n g components on b u i l d i n g envelopes t h e heat consumption o f those b u i l d i n g s can be s i g n i f i c a n t l y reduced.
1
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INTRODUCTIONI f t r a n s p a r e n t thermal i n s u l a t i o n i s used i n combination w i t h s o l i d e x t e r i o r w a l l s , t h e i n s u - l a t i n g e f f e c t s may be enhanced by h e a t gains from s o l a r r a d i a t i o n Ill. F i g u r e 1 shows t h e f u n c t i o n o f a t r a n s p a r e n t i n s u l a t e d mass w a l l i n a conceptual diagram: Solar r a d i a t i o n w i l l be p a r t i a l l y t r a n s m i t t e d through t h e i n s u l a t i n g l a y e r and i s absorbed a t t h e e x t e r i o r surface o f t h e w a l l behind t h e i n s u l a t i n g l a y e r . Due t o t h i s attached l a y e r , a h i g h percentage o f t h e absorbed energy i s conducted i n t o t h e i n t e r i o r l e a f 121. Depending on t h e i n s o l a t i o n t h e w a l l w i l l heat up and a temporary heat t r a n s f e r i n v e r s i o n from t h e o u t s i d e t o t h e i n s i d e can be achieved. The t r a n s p a r e n t i n s u l a t e d w a l l a c t s as a heater.
Solar radiation
~nsu~atidn layer
F i g . 1
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Transparent w a l l i n s u l a t i o n i n a conceptual diagram.Organic p l a s t i c m a t e r i a l s such as polycarbonat honeycombs, c a p i l l a r y s t r u c t u r e s o r a c r y l i c foam, b u t a l s o i n o r g a n i c g l a s s f i b e r m a t e r i a l s or aerogel a r e a p p r o p r i a t e m a t e r i a l s t o be used i n t r a n s p a r e n t i n s u l a t i o n systems 131. To f u r n i s h those products w i t h s u f f i c i e n t mechan- i c a l p r o p e r t i e s and t o ensure a good weather and f i r e p r o t e c t i o n behaviour t h e y a r e used be- tween double g l a z i n g s 141. F i g u r e 2 g i v e s a schematic p r e s e n t a t i o n o f a double glazed window f i l l e d w i t h granulated SiO, aerogel. I n t h i s paper r e s u l t s o f experimental i n v e s t i g a t i o n s performed on a w a l l c o n s t r u c t i o n o f t h i s t y p e w i l l be presented.
-Glass pane
-Granulated Si02
-
aerogel-Edge sealing
Fig. 2 - Schematic p r e s e n t a t i o n of a double-glazed window f i l l e d w i t h granular Si02 aerogel.
Experimental i n v e s t i g a t i o n s o f a g l a z i n g u n i t mounted t o a 0.2 m heavy concrete w a l l were c a r r i e d o u t a t t h e t e s t s i t e o f our i n s t i t u t e i n S t u t t g a r t . The aerogel window (see F i g . 2) c o n s i s t s o f two 4 mm g l a s s panes w i t h conventional rim s e a l i n g . The space o f 12 mm between t h e panes i s f i l l e d w i t h granulated aerogel 15, 61.
The U-value o f t h i s g l a z i n g u n i t i s 1.1 W/(m2K), i t s d i f f u s e t r a n s m i t t a n c e i s 0.5, and t h e o v e r a l l U-value o f t h e w a l l c o n s t r u c t i o n i s 0.97 Wl(m2K). The t h i c k n e s s o f t h e i n s u l a t i n g l a y e r i n f l u e n c e s b o t h t h e s o l a r t r a n s m i s s i o n and t h e thermal transmittance. F i g u r e 3 shows t h e s o l a r t r a n s m i t t a n c e o f granulated aerogel between g l a s s panes o f 2 mm t h i c k n e s s as com- pared t o PMMA foam and c a p i l l a r y s t r u c t u r e m a t e r i a l . O f course, t h e t r a n s m i t t a n c e i s depend- i n g on t h e porous diameter i n t h e case o f foam and, accordingly, on t h e diameter o f t h e ca- p i l l a r i e s . The values given i n F i g s . 3 and 4 are based on diameters o f 5 mm f o r t h e aerogel granules, o f 5 mm f o r t h e foam pores and o f 1.7 mm f o r t h e c a p i l l a r i e s .
For a l a y e r o f 30 mm t h e d i f f u s e s o l a r t r a n s m i t t a n c e o f t h e aerogel window i s 0.34. PMMA foam a t t a i n s higher r a t e s , namely 0.46, and c a p i l l a r y s t r u c t u r e has a value o f 0.63, f o r t h e same l a y e r t h i c k n e s s (see F i g . 3). A t r a n s m i t t a n c e o f 0.5 on t h e o t h e r hand corresponds t o a U- value o f about 0.4 Wl(m2K) f o r t h e c a p i l l a r y s t r u c t u r e m a t e r i a l , o f 1.2 f o r t h e aerogel win- dow, and o f 1.7 Wl(m2K) f o r PMMA foam.
F i g . 4
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R e l a t i o n between d i f f u s e s o l a r t r a n s - m i t t a n c e and U-value o f a window f i l l e d w i t h g r a n u l a r aerogel as compared t o PMMA foam and c a p i l l a r y s t r u c t u r e m a t e r i a l l a y e r s 141. (The aerogel window panes a r e 2 mm t h i c k . )F i g . 3
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E f f e c t s o f pane t h i c k n e s s on t h e s o l a r t r a n s m i t t a n c e o f a window f i l l e d w i t h granulated aerogel as compared t o PMMA foam and c a p i l l a r y s t r u c t u r e m a t e r i a l l a y e r s 141. (The aerogel win- dow panes a r e 2 mm t h i c k . )-0 10 20 30 LO 50 60
thickness Cmml
1.0
0.8
0.6
0.2
o----
0 0.4 0.8 1.2 1.6 2.0 2L
U-value [ ~ l ( r n ~ ~ ) l
By replacing the a i r i n the intermediate space w i t h argon gas or by evacuating it, t h e u n i t ' s thermal resistance may be f u r t h e r improved. Test s i t e and t e s t conditions are indicated i n Fig. 5. The aerogel window i s one o f Seveicl south facing w a l l elements w i t h transparent i n - s u l a t i o n 17/ which had been under i n v e s t i g a t i o n . The c l i m a t i c conditions p r e v a i l i n g during the measurement cycle from February t o May 1986 are characterized by outdoor a i r temperatures between -16 O C and more than 30 O C . Measured climate parameters included i n s o l a t i o n (up t o 6.7 kWh/(m2d)), degree days (1712 f o r the measurement period, 3761 Kd f o r the heating pe- r i o d ) , and indoor a i r temperatures (about 20 OC).
Fig. 5
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South f a c i n g w a l l elements w i t h F i g . 6-
D e t a i l e d view o f the glazing u n i t d i f f e r e n t transparent i n s u l a t i n g materials w i t h granular Si02 aerogel.and an opaque insulated reference w a l l ( l e f t element). The w a l l element w i t h the aerogel window i s placed a t r i g h t .
The element presented i n the r i g h t p a r t o f Fig. 5 contains granulated SiO, aerogel. Figure 6 shows t h i s component, which reminds a l i t t l e o f a structured marble plate, i n greater d e t a i l .
3
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EXPERIMENTAL RESULTSI n the f o l l o w i n g , the most s i g n i f i c a n t r e s u l t s concerning thermal performance and energy b a l - ance w i l l be demonstrated and discussed. I n Fig. 7, the mean monthly e x t e r i o r w a l l surface temperatures o f the transparent insulated t e s t w a l l and the range of f l u c t u a t i o n during the measurement period are contrasted w i t h the outdoor a i r temperature. Values f o r the averaged outdoor a i r temperature o f each month are between -6 O C and 15 O C . During the monitoring pe- r i o d temperatures f l u c t u a t e d between a minimum o f -16 O C and a maximum o f 31 O C . Average tem- peratures recorded f o r the w a l l ' s e x t e r i o r surface behind the i n s u l a t i n g layer varied between
19 O C and 25 O C . I n terms o f hourly mean values, the lowest temperature was 12 O C and the
highest one was 44 O C .
Outdoor air Exterior surface
-20
-
Feb. Mar. Apr. May -20-l:m/
Feb. Mar. Apr. May
F i g . 7
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Mean monthly e x t e r i o r w a l l s u r f a c e temperatures o f t h e transparent i n s u l a t e d t e s t w a l l ( r i g h t ) as compared t o t h e outdoor a i r temperature ( l e f t ) a r e i n d i c a t e d by t h e hatched area.F i g u r e 8 presents mean monthly temperatures f o r t h e i n t e r i o r w a l l surfaces o f t h e transparent i n s u l a t e d t e s t w a l l i n comparison t o t h e reference w a l l having an opaque i n s u l a t i o n o f t h e same U-value. The monthly mean temperatures o f t h e t e s t w a l l range from 19 O C t o 23 O C , those o f t h e reference w a l l from 16 O C t o 20 O C . I n February and March, t h e temperatures read a t t h e i n t e r i o r s u r f a c e o f t h e reference w a l l f l u c t u a t e between 15 O C and 19 O C , which i s a much lower l e v e l than t h a t recorded f o r t h e t r a n s p a r e n t i n s u l a t e d t e s t w a l l . I n f a c t , t h i s t e s t w a l l experiences temperatures between 15 O C and 30 O C as e a r l y as February and March.
Test wall Reference wall
(transparent) (opaque)
.
-
Feb. Mar. Apr. May
F i g . 8
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Mean monthly i n t e r i o r w a l l s u r f a c e temperatures o f t h e t r a n s p a r e n t i n s u l a t e d t e s t w a l l ( l e f t ) as compared t o t h e opaque i n s u l a t e d reference w a l l ( r i g h t ) . The data spread a g a i ni s i n d i c a t e d by hatching.
C 4 - 1 0 4 REVUE DE PHYSIQUE APPLIQUEE
F i g u r e 9 i l l u s t r a t e s t h e i n f l u e n c e o f c l i m a t e on t h e e n e r g e t i c behaviour o f b o t h t e s t and r e f e r e n c e w a l l . I n order t o consider t h e i n f l u e n c e o f i n s o l a t i o n as w e l l as t h e outdoor a i r temperature, a new f a c t o r has been defined:
I n s o l a t i o n
T h i s i s t h e so c a l l e d degree-day i n s o l a t i o n f r a c t i o n . By analogy w i t h t h e d e f i n i t i o n o f t h e U-value (thermal t r a n s m i t t a n c e c o e f f i c i e n t ) , an e f f e c t i v e U-value i s d e f i n e d t h a t t a k e s heat g a i n e f f e c t s from s o l a r r a d i a t i o n i n t o account:
Heat f l u x d e n s i t y
Ueff = D i f f e r e n c e between indoor and ambient a i r temperature
-
1-
YN
2
E a3 3d
9
-1I
3
? - 2 .- +
%
- 3- L
0 0.1 0.2 0.3 0.L 0.5
degree-day insol. frac. [ k ~ h / ( r n ~ ~ d ) l
r i g . 9
-
Measured e f f e c t i v e U-values o f t h e t e s t w a l l w i t h and w i t h o u t s u r p l u s h e a t as a f u n c t i o n o f c l i m a t e (degree day i n s o l a t i o n f r a c t i o n ) . I n a d d i t i o n t o t h e o v e r a l l U-value o f t h e w a l l c o n s t r u c t i o n s , t h e measured e f f e c t i v e U-value o f t h e opaque i n s u l a t e d reference w a l li s given (see c i r c l e t o p l e f t ) .
The a b s o r p t i o n o f s o l a r r a d i a t i o n r e s u l t s i n a temporary i n v e r s i o n o f heat t r a n s f e r w i t h i n t h e t r a n s p a r e n t i n s u l a t e d w a l l and, a t l e a s t , i n a r e d u c t i o n o f heat losses as compared t o t h e opaque i n s u l a t e d r e f e r e n c e w a l l . F i g u r e 9 shows t h a t t h e e f f e c t i v e U-value o f t h e r e f e r - ence w a l l i s p r a c t i c a l l y independent o f s o l a r i n p u t .
There a r e two l i m i t i n g values, namely one c o n s i d e r i n g s u r p l u s heat, and one d i s r e g a r d i n g s u r - p l u s heat. I f t h e r e a r e o n l y heat losses through t h e w a l l , t h e upper l i m i t o f e f f e c t i v e U- values w i l l be obtained. I n F i g . 9 these values a r e i n d i c a t e d i n t h e graph " w i t h o u t s u r p l u s h e a t " . But even i f t h e r e i s no s u r p l u s heat, t h e e f f e c t i v e U-value i s c l e a r l y reduced as com- pared t o t h e reference w a l l w i t h opaque i n s u l a t i o n . Taking temporary s u r p l u s heat i n t o ac- count, t h e e f f e c t i v e U-values may become negative. The heat gains then a r e exceeding t h e heat losses through t h i s w a l l . I n c l u d i n g s u r p l u s heat, t h e e f f e c t i v e U-value reached -3 Wl(m2K) i n May. The mean value o f t h e measurement p e r i o d amounts t o -0.5 Wl(m2K).
4
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CONCLUSIONSI n a d d i t i o n t o t h e thermal i n s u l a t i n g e f f e c t s achieved by t r a d i t i o n a l opaque i n s u l a t i o n , t r a n s p a r e n t i n s u l a t i o n s u p p l i e s considerable passive s o l a r gains when used as a thermal i n s u l a t i o n o f mass w a l l s . With a double g l a z i n g u n i t f i l l e d w i t h granular s i l i c a aerogel and mounted onto a 0.2 m heavy concrete w a l l , t h e e x t e r i o r s u r f a c e temperature has a maximum o f 44 OC, w i t h a maximum o f 30 OC measured a t t h e i n t e r i o r surface. The U-value o f 0.97 Wl(m2K) w i l l be e f f e c t i v e l y reduced t o a value o f - 0 . 5 Wl(m2K) averaged over t h e m o n i t o r i n g p e r i o d (February through May). To a v o i d overheating i n summer, i t i s necessary t o p r o v i d e some s o l a r p r o t e c t i o n . By u s i n g these g l a z i n g elements on b u i l d i n g envelopes t h e heat consumption o f those b u i l d i n g s can be s i g n i f i c a n t l y reduced. Even d u r i n g t h e h e a t i n g p e r i o d t h e r e a r e many days w i t h s u r p l u s heat.
ACKNOWLEDGEMENT
We a r e g r a t e f u l t o BASF AG, Ludwigshafen (FRG), f o r p r o v i d i n g t h e t e s t m a t e r i a l . T h i s work was supported by BMFT 03 E-8411-A and s e v e r a l German companies (see Ref. 141).
REFERENCES
Ill Schreiber, E.; Boy, E. and Bertsch, K.: Aerogel as a Transparent Thermal I n s u l a t i o n M a t e r i a l f o r B u i l d i n g s . Proc. o f t h e F i r s t I n t e r n a t i o n a l Symposium on Aerogels. Springer Ver-
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