Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
10~734~i
The pres~nt invcntion relates to a solar energy
r,' collection system. This application is a division of
copending Canadian application Serial No. 293,819 filed
December 23, 1977.
In our prior Canadian patent application Serial
No. 275,382, (now Canadian Patent No. 1,027,820) there
is described a solar energy collector compriSing an outer
evacuated envelope having an upper transparent surface
to admit light rays to the envelope and a two-way tube
extending in the envelope from one end thereof towards
- the other and having a selectively absorbing surface for
; selectively absorbing energy having predetermined wave-
lengths and rejecting other wavelengths.
An elontage reflector surface is located
,~ internally of the envelope and is arranged to reflect
light received through the upper transparent surface onto
the two-way tube. The locus of the reflector surface
is designed to achieve maximum efficiency and is the shape -
,' required to ensure that all incident rays reoeived into
;; 20 the envelope through the upper transparent surface)within
the acceptance angle determined by the equation:
~' ~ . C = 1
;' sin~
:''.', . "
where C is the concentration ratio (i.e., the ratio of
the transverse width of the upper transparent surface to
the outer circumference of the tube) and ~ is the -
~';' acceptance angle, are concentrated on the two-way tube
~`' while rays outside the acceptance angle are reflected.
, .. .
The shape of the reflector surface determined
by the above equation represents the maximum efficiency
. ,
, of the collector. In accordance with the present
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.. invention, the locus o~ the rePlector surface i.s the shape
required to ensure that from ahout 75% of, preferably
. from about 90% of, to less than the maximum efficiency
of the collector is realized. Although there is a decrease
in efficiency, advantages in truncation o the reflector
. surface, with consequentially reduced material costs,
result.
. . .
Accordingly, the present invention provides a
. solar collector comprising: an outer evacuated elongate
;- 10 envelope having an upper transparent surface to admit
~- light rays to the envelope, a tube extending in the
envelope from one end thereof towards the other for
conveying fluid to be heated into the collector and for
removing heated fluid from the collector, the tube
comprising a concentrically-arranged pair of inner and
_, ,
:~.- . outer tubes extending from one end of the envelope
~ substantially to the other and an end closure at the other
,;:;,;
. end constructed to define a flow channel from the inner
to the outer tube or vice versa, whereby fluid to be
... ` 20 heated enters the collector from the same end as heated
................ fluid is removed from the collector, a selectively
~` absorbing surface on the tube for selectively absorbing
., ~ .
:.; . energy having predetermined wavelengths and rejecting
.. . .
.. other wavelengths, and an elongate reflector surface
. .~ .
located internally of the envelope and arranged to .
; reflect light received through the transparent surface
onto the tube, the upper transparent surface and the tube
., ,.~
being dimensioned to provide a concentration ratio which
is the ratio of the transverse width of the upper trans-
~ . 30 parent surface to the outer circumference of the tube and
`. has a value greater than about 0.5, the locus of the
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~ .
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reElec~.or sur~ace b~iny ~he sh~p~ requ:ir~d to ~nsure that :
from about 75% oE to less than -the maximum eficiency of
the coll~ctor is realized, the maximum efficiency being .
~; provided by the shape required to ensure that all incident .
rays received into the envelope through the upper trans-
~ parent surface within the acceptance angle determined by ~ .
;. the equation~
C=
sin~
where C is the concentration ratio and ~ is the acceptance .
angle, are concentrated on the tube while rays outside
the acceptance angle are reflected. - :
The invention is described further, by way o
illustration, with reference to the accompanying drawings, . -.
wherein: .
, . ~ . , .:
; Figure 1 is a perspective view, with parts cut
away, illustrating a solar coIlector constructed in accor- .
., dance with this invention; and .
Figure 2 is a section taken on line 2-2 of ~.
. 20 Figure 1. .-
.. ~ In the drawings, a solar collector 10 is pro- ¦
~ vided in the form of an elongate, integral generally tub- ~ 1
r::; ular element, preferably fabricated wholly of glass or .-
. other convenient material, having a tubular end portion
~ 12.
A tube 14 extends generally axially of the
collector 10 and the portion of the tube 14 extending
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into the collector 10 beyond the tubular end portion
12 is surrounded by an axially extending parallel tubular
.. 30 portion 16 integral with and of the same diameter as the
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~7946
:
tubular end portion 12 at one end and closed at the other
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beyon~ the ~xtremi ty oE th~ ~ube ~ o d~E:Lne a :Eluid .Elow
path ~hrough the collector 10. rrhe ~luid flow pakh extends
~ internal~y ~ the tube 14 and then between the internal sur-
;~ face of the tubular portion 16 and the outer surface o~ the
tube 14, or vice versa, depending on the direction of fluid
flow.
An outer envelope 18 surrounds and encloses the
tube 14 and the tubular portion 16 and is integral with
the tubular end portion 12. The space 20 between the ~uter
envelope 18 and the tubular portion 16 is evacuated.
The outer envelope 18 is contoured as shown in the
sectional view of Figure 2 and has a relatively flat top
~' surface portion 22 to allow entry of sun rays into the
collector 10.
The remainder of the inner surface of the outer
envelope 18 is coated with a highly reflective material 24,
such as silver, to reflect sun rays entering the collector
10 through the top surface 22 and impinging on the envelope
inner surface.
, :~
The tubular portion 16 has a high absorbance at all
incident angles in the spectral wavelength of 0 to 3 x 10 6
meters. In order to decrease heat losses, the absorbance for
higher wavelengths should be low. The selective absorbance
, may be achieved by coating the outer surface of the tubular;~ .,
~` portion 16 with a suitable material, such as chrome black.
The circular cross section of tube 16 may be replaced by
..~1
~ any other desired cross-sectional shape.
., .
In the collector 10, the concentration ratio (C)
refers to the relative dimensions of the radiation-receiving
portion and radiation-absorbing portion of the collector,
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while the acceptance ang:Le (~) reEexs to ~he angle withln
which all rays entering the collector 10 through the radia-
tion-receiving portion are absorbed by the radiation- ¦
absorbing por~ion of the collector while rays entering the
~- collector through the radiation-receiving portion outside
that angle are reflected.
Referring to Figure 2, the concentration ratio (C)
; of the collector is determined by the ratio: ¦
C = Entrance Aperture Width = A
Absorber Tu~e Circumference 2 ~ R
~ lO The acceptance angle (~) is the angle to the vertical
; axis within w~ich all rays entering the collector through
the upper surface 22 are absorbed by the two~way tube 16 while
.. , ,
rays outside that angle are reflected back without being
absorbed. The limiting condition far acceptance of rays
',l for absorption is a ray which is reflected by the reflecting
:,. .
surface to pass tangentlally to the two-way tube 16, as
illustrated.
; In a collector of maximum efficiency, the
~'! acceptance angle (0) is determined by the concentration
ratio (C~ in accordance with the equation:
. i ~C =
; sin
and the locus of the reflecting surface 24 of the collector
;~.
is the shape corresponding to that equation.
It will be seen from the above equations that,
as the concentration ratio (C) increases, the acceptance
angle (~) decreases. The value of the acceptance angle
will determine the length of time during a given day when
.. . .
the collector will absorb light rays, assuming that the
collector is located in a fixed relationship with respect
to the sun mavement. The value of the concentration ratio
will determine the temperature rise attainable in the
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7~34~
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two-way tube during the time that rays are accepted
:~ within the acceptance ~ngle, with an increase in concentra-
tion ratio leading to an increase in temperature under
otherwise flxed conditions.
; The minimum concentration ratio for the
-` ` collector is about 0.5 and the upper limit of concentration .
ratio for a fixed location system is about 10. If the
~ collector is mounted to track the sun's movement on a . .
' daily basis or if the sun's rays can be concentrated :~
~ 10 within the narrow acceptance angle which exists at these :~
`i high concentration ratios, then the concentration ratio
: may exceed 10, although it will rarely exceed 50.
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i:l Preferably, the concentration ratio is about
`~~ 1.0 to about 3.0, most preferably about 1.5 to about 2.0, .
which provides a good balance of acceptance angle and :
' concentration ratio, so that the collector has a
sufficiently wide acceptance angle to absorb rays over a .
1 long period of daylight hours, while at the same time ~
:: providing a good heating effect on the fluid flowing .
.. 1 . .
through the collector. .
If the physical height of the body portion 18
~; is decreased without otherwise altering the shape of the
, reflector surface 24, as shown in the modification of
Figure 2 wherein the dotted outline represents the locus
,~; i
~` of maximum efficiency and the solid outline represents ~:
. ; .
; the decreased height body, the concentration ratio is
decreased and thus leads to a less than maximum efficiency
of collector 10.
. ~
: Since, however, the upper portions of the reflec- !
.. 30 ting surface 24 adjacent the upper surface 22 are almost
, ,
parallel and have only a minor effect on the rays which
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are absorb~d by the tube 16, the loss in e~Elciency nee~
~ only be minor, while the material saving achle~ed thereby
: ~` is considerabl~. ¦
In this invention, the maximum loss of e~ficiency
rom ideal conditions is about 25~, while preferably the loss
o~ ef~iciency tolerated on truncation is less than about
.
10%. -
The outer envelope 18 of the collector preferably
is comprised of (1) an upper surface portion formed of
~, 10 glass and constituting the transparent surface and (2)
a lower body portion constructed of vitreous ceramic :
. .~ . ,
's` material formed from clay and various fluxes. Vitreous.
, ceramic materials are inexpensive and readily available,
:
and can be ~ormed into shaped objects by molding or
, axtrusion, making them ideal for formation of an integral
molded or extruded lower body portion.
~ The collectors of this invention may achieve a
,I photovoltaic function by producing an electrical output
from collected solar energy. The two-way tube 16 may be
-coated with light energy actuable electricity generating
material layers which communicate through suitable elec- ¦
' trical connection to exterior of each collector 10.
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