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The heliodon
THE HELIODON by
G.
P. Mitalas ,,4. cj7I
I
I
December 1964 --.- .- -- D I V I S I O N OF B U I L D I N G R E S E A R C H.
N A T I O N A L R E S E A R C H C O U N C I L a O T T A W A.
C A N A D A -- --THE HELIODON
by
6 . P. Mitalas
The c u r r e n t p r a c t i c e of using l a r g e a r e a s of g l a s s in e x t e r i o r w a l l s a g g r a v a t e s the p r o b l e m of coping with s o l a r r a d i a t i o n on buildings.
S o l a r control by outside s h a d e s i s one of the m e t h o d s t h a t can be advantageous in s o m e applications. T h e design of outside s h a d e s o r the d e t e r m i n a t i o n of shading b y s u r r o u n d i n g o b j e c t s . r e q u i r e s knowledge of the g e o m e t r i c r e l a t i o n s between the sun, building and shading s u r f a c e . In the c a s e of a s i m p l e shade, such a s the v e r t i c a l o r h o r i z o n t a l projection f r o m a v e r t i c a l wall, the a r e a shaded by the p r o j e c t i o n c a n be d e t e r m i n e d by calculation, utilizing s o l a r c h a r t s , slide r u l e s o r t a b l e s , developed specifically for the purpose.
When the complexity of the building and s h a d e s i n c r e a s e s , t h e s e methods become tedious and t i m e consuming. It is then advantageous t o u s e m o d e l s to d e t e r m i n e the shadow l i n e s . In t h i s c a s e , i t i s only n e c e s s a r y t o position the m o d e l r e l a t i v e t o a b e a m of light in the s a m e way a s the r e a l object will s i t r e l a t i v e to the s u n ' s r a y s . A heliodon i s a s i m p l e device for positioning a model with r e s p e c t t o a light beam ( F i g u r e 1).
The angle a t which the sun s t r i k e s the s u r f a c e of a building depends on :
(1) t i m e of day ( 2 ) date
( 3 ) latitude of the building s i t e
(4) orientation of the building with r e s p e c t t o north.
The heliodon h a s a d j u s t m e n t s for a l l four p a r a m e t e r s and when t h e y have a l l been s e t t o the p r e s c r i b e d values, the model i s in the d e s i r e d position r e l a t i v e t o the beam of light.
A heliodon h a s been designed a t DBR; F i g u r e 1 is a photograph of i t and the working drawings a r e given in F i g u r e s 2 , 3 , 4, and 5.
It should be noted t h a t the graduation m a r k s for latitude and t i m e - o f - y e a r s c a l e s a r e not given on the drawings. T h e s e m a r k s c a n be i n s c r i b e d
a c c u r a t e l y a f t e r the heliodon i s a s s e m b l e d . T h e latitude s c a l e is obtained b y setting the angle
90-rp
( w h e r e cp i s the latitude angle), using a suitable p r o - t r a c t o r , and m a r k i n g the cr, value opposite the latitude a d j u s t m e n t s l i d e r on t h e latitude s c a l e face. T h i s p r o c e d u r e is r e p e a t e d until the latitude s c a l e i s- 2 -
complete for the r a n g e of latitude a n g l e s that c a n be s e t with t h e latitude a d j u s t m e n t a s s e m b l y .
A s i m i l a r p r o c e d u r e can be u s e d for m a r k i n g the two t i m e -of - y e a r s c a l e s
-
one for s p r i n g and s u m m e r , and the other for autumn and w i n t e r . T h e difference i s that the angle 6 i s s e t and the t i m e of year i s noted opposite the time-of-year a d j u s t m e n t s l i d e r s . T h e v a l u e s of 6 v e r s u s t i m e of y e a r a r e given in T a b l e I.L o c a l s o l a r t i m e . i s used t o s e t the t i m e of day on the h e l i ~ d o n . T o obtain the l o c a l s o l a r t i m e c o r r e s p o n d i n g t o the l o c a l s t a n d a r d t i m e , the following equation c a n be u s e d :
l o c a l s o l a r t i m e = l o c a l s t a n d a r d t i m e
-
KA+
B w h e r eA
=
Longitudinal difference in d e g r e e s between the longitude of the s i t e and the longitude a t the c e n t r e of the l o c a l s t a n d a r d t i m e zoneB = Equation of T i m e . T h e v a l u e s of Equation of T i m e a s well a s t h e d e c l h a t i o n angle v e r s u s t i m e of y e a r a r e given in T a b l e I for the year 1964. F o r shadow angle evaluation, t h e s e v a l u e s c a n be u s e d for a n y y e a r .
OPE RAT ION
The building m o d e l is p l a c e d on the mounting p l a t f o r m and
o r i e n t e d with r e s p e c t to the c o m p a s s m a r k i n g s . The latitude angle i s s e t by positioning the latitude a d j u s t m e n t s l i d e a t the r e q u i r e d latitude angle
graduation.
T h e a p p r o p r i a t e t i m e - o f - y e a r s c a l e t o be used ( s p r i n g and s u m m e r ; autumn and w i n t e r ) depends on t h e b a s e p l a t e support p o s i t i o n :
open position ( b a s e p l a t e a t 2 3 . 5" t o horizontal)
-
s p r i n g and s u m m e r s c a l e folded position ( b a s e p l a t e h o r i z o n t a l )-
a u t u m n andwinter , s c a l e . Once t h e b a s e p l a t e s u p p o r t i s positioned, the t i m e of y e a r i s s e t by positioning the declination a d j u s t m e n t s l i d e r s a t the r e q u i r e d date m a r k .
- 3 -
The time of (lay i s s e t simply by rotating the,mount;ing. platforin with respect to. the base p l a t e . When the light beam i s parallel to th.e mounting plate surface,, this i s the time when the sun i.s .just below the horizon.
The light source must be placed s o that the centreline of the light beam i s p a r a l l e l to the 6
=
0" plane (i. e . the light beam must be p a r a l l e l tothe surface supporting the heliodon). In addition the heliodon must be
oriented so that the light beam i s perpendicular to the e a s t -west line when the time of day i s s e t at 1 2 or 2 4 h r .
One of the difficulties common to a l l heliodons i s the generation of a beam of p a r a l l e l light r a y s large enough t o illuminate all the model. Small spotlights (approximately 6-in. diameter) a r e usually used for convenience but the light beam from these must diverge t o cover an a r e a a s l a r g e a s the model. The simplest way to reduce the divergence of the light r a y s a r r i v i n g a t the model i s to place the light source a s far a s possible f r o m the model, since the resulting e r r o r in the shadow angle at any particular point on the model is related to the r a t i o of the distances from the point t o the c e n t r e of the light beam (assumed t o be the c e n t r e of the mounting platform), and the distance from the model to the light source focal point. F o r example, i f the point on the model i s 1 ft from the centreline of the light beam and the light source i s
18 ft away, and the focal point 2 ft further from the model, the divergence a t
this point i s approximately 3 degrees.
The intensity of the light source required to give sharp shadow definition depends on the distance between the model and the light source and the divergence of the light r a y s . Satisfactory r e s u l t s can be obtained using a simple slide projector or photospot light a t a distance of 30 ft.
Example P r o b l e m
I I
It i s required to determine the shaded a r e a s of a building located in Ottawa (latitude
=
45. 5 " , longilude = 75. 5" ) during the day of June 15. The building faqade i s facing east. It i s assumed that a suitable building model i s available.P r o c e d u r e :
(1) P l a c e the heliodon on a horizontal surface.
( 2 ) Set the latitude adjustment slider a t 45.
5"
mark.( 3 ) Set the time-of-year adjustment s l i d e r s a t June 15 m a r k s . (4) Set the b a s e plate support in the open position since
( 5 ) P l a c e the light s o u r c e a s f a r away a s p o s s i b l e f r o m the heliodon and position the c e n t r e of t h e light on a l e v e l with t h e c e n t r e of the mounting p l a t f o r m .
( 6 )
P l a c e a s t r a i g h t r o d p e r p e n d i c u l a r t o and in t h e c e n t r e of t h e mounting p l a t f o r m .( 7 ) S e t t h e t i m e of day at 1 2 noon and position the
heliodon s o that the shadow of the r o d i s p a r a l l e l to the n o r t h -south line.
(8) Place the building m o d e l on the mounting p l a t f o r m and
o r i e n t i t with r e s p e c t to the c o m p a s s m a r k i n g s .
( 9 ) T h e shaded a r e a s of t h e building a t a given s o l a r t i m e
of day a r e obtained by s e t t i n g t h i s t i m e on the heliodon and o b s e r v i n g t h e shaded a r e a s on t h e model.
T A B L E I
Declination Eq. of T i m e Angle
6
Month Day Deg. Min. Min. S e c . Deg. Min.J a n u a r y 1 -23
6
-
3 00 0 2 41 5 - 2 1 2 0 -
