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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a fibrous wall material for cell
structures useful in solar energy collectors and more particularly to
solar energy collectors having fibrous walls that are substantially
transparent to solar radiation and substantially impervious to long-wave
radiation.
Solar energy collectors for converting the energy of solar radiation into
heat as compared to solar cells, which utilize photoelectric effects,
consist of a solar radiation absorber to absorb the radiation which is
absorbed as completely as possible, and a suitable arrangement whereby
heat is conducted from the absorber to the heat storage unit or directly
to the device which utilizes the heat. The heat is generally carried away
by a flowing medium (gas or liquid).
The absorber which is heated by the solar radiation, not only gives off its
heat to the transporting medium, but also loses heat to the surroundings.
Such undesired losses occur with both concentrating collectors and flat
collectors.
With flat collectors, the side which is farthest from the incident solar
radiation can be easily protected against heat losses. For example,
conventional insulating materials, such as glass and rock wool or foam
plastic materials, having a suitable thickness, provide good heat
insulation at a low cost. It is more difficult to protect that side of the
absorber which is exposed to the solar radiation against heat losses.
Heat-insulating means which are arranged on this side of the absorber,
must, in fact, satisfy the condition that the radiation is able to pass
through the heat-insulating arrangements as far as possible, unhindered.
Thus, the side of the absorber receiving the solar radiation should be
substantially transparent for solar radiation.
Heat losses are caused by heat conduction, convection and radiation
exchange. Steps which are taken for suppressing these heat losses
frequently are only concerned with one of said forms of heat transfer, and
sometimes, more than one of these forms simultaneously.
Heat losses of the solar collectors due to radiation exchange can be
suppressed by various methods. Frequently, selectively reflecting layers
or coatings are frequently used as absorbers. These layers absorb the
solar radiation sufficiently well, but, on the other hand, only emit
long-wave infrared to an insignificant degree. Coatings on transparent
covering sheets or panes, which are transparent for the solar radiation,
but are able to reflect long-wave infrared radiation, act in a similar
manner. For example, if such a layer or coating is on the covering pane,
on the side facing the absorber, the radiation emitted by the absorber is
reflected on the layer and is again absorbed by it. Single or multiple
covering panes, which are transparent for solar radiation, but absorb
long-wave infrared, are not quite as effective as the measures which have
been described above. If an intermediate space is subdivided by an
additional pane, the heat transfer in this intermediate space is
approximately halved due to radiation exchange.
Heat losses due to heat conduction and convection are closely related to
one another with regard to solar energy collectors. The side of the
absorber which faces the solar radiation is usually bounded by air. This
layer of air conducts heat to the surroundings. It is not sufficient to
make this gas layer so thick that the heat losses due to conduction become
negligibly small. The convection, which likewise quickly increases, as the
thickness of the gas layer increases, leads to the sum of the heat
transfer fractions of conduction and convection being almost independent
of the gas layer thickness, once a certain thickness is exceeded.
Thus, for example, with flat collectors having several covering panes which
are transparent for solar radiaiton, the distance between the absorber and
the pane disposed thereabove, or between two panes, and having a thickness
of about 15 mm, has no influence on the heat-insulating properties of the
arrangement. Any increase in the thickness of the gas layers results in an
increase in convection.
One method frequently used for suppressing heat conduction and convection
is to enclose the absorber in a vessel which permits the solar radiation
to pass through to the absorber and is capable of being evacuated. Below a
certain pressure, the convection is reliably suppressed. If the pressure
is still further reduced, a point is then reached wherein a further
decrease in the pressure reduces the heat conduction. Vessels which are
capable of being evacuated must, however, be able to withstand the
atmospheric pressure, and consequently this can only be achieved at great
expense with flat collectors.
All methods so far known have been used in the widest possible range of
application, both in connection with concentrating collectors and flat
collectors.
Concentrating collectors with selectively reflecting absorber layers,
selectively transmitting layers on the covering panes, or also both, are
for example known. The enclosing vessels are often evacuated to a greater
or lesser degree.
With regard to flat collectors, many arrangement with selectively
reflecting absorber layers under single or multiple sheet or pane
coverings have been tested.
Inherent in all of these combinations are disadvantages which cannot, in
principle, be overcome by the measures which have been set forth herein.
Thus, at least one transparent covering is already required for keeping
rain and dirt away from the absorber. Each additional covering, although
desirable for heat insulation purposes, does however increase the
absorption and reflection losses of the solar radiation in its passage
through the covering to the absorber.
Selectively reflecting layers are expensive and usually present an
absorption coefficient which is far removed from the optimum. Moreover, at
relatively high temperatures, these layers are often unstable.
In order to avoid or minimize the aforementioned disadvantages, it has been
proposed in the prior art to provide honeycomb-like structures between the
absorber and the transparent cover sheets. If the shape and size and also
the wall material are suitable chosen, then both the radiation exchange
and the convection are reduced or almost completely suppressed.
The honeycomb walls generally stand perpendicular on the solar radiation
absorber. HOTTEL.sup.1 previously showed that the radiation exchange
between bottom and top of such honeycombs or cells is dependent upon the
form or shape thereof and on the ratio between average diameter D and the
height H of the cell. For cells having walls which absorb long-wave
infrared, the radiation exchange -- as compared with unhindered exchange
-- is reduced by a factor F.
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Approximately the followings values apply:
F = 0.52 0.36 0.27 0.22 0.19 0.10
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H/D = 1 2 3 4 5 10
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.sup.1 HOTTELL, Mech.Eng. 52 (1930) 7, pages 699-704. HOTTELL,
Am.Soc.Mech.Eng. Paper IS-55-6, Vol. 55, (1933) pages 39-49.
i.e., a cell structure of which the mean cell diameter amounts to only a
tenth of the cell height, suppresses the heat transmission due to
radiation by the factor 10, pre-supposing that the material of the cell
wall absorbs long-wave infrared.
If the mean diameter of a single honeycomb is chosen small enough, then the
convection is also suppressed. Depending on temperature difference and
honeycomb height, it is possible to find a diameter below which the
convection is completely suppressed..sup.2 For a temperature difference of
50.degree. C. between bottom and top of the honeycomb, it was possible to
show that, below a cell diameter of 1 cm., the convection was completely
suppressed..sup.3
.sup.2 TABOR, Solar Energy, Vol. 11, pp. 549,552, Pergamon Press, 1969.
.sup.3 HOLLANDS, Solar Energy, Vol. 9, No. 3, 1965, pp. 159-164.
In the foregoing, only the properties of such cell structures have been
described, which properties are of importance for the heat-insulating
behavior thereof. In addition to having the properties as described, the
honeycombs must, above all, allow the solar radiation to reach the
absorber. The material of the cell wall must therefore be transparent or
highly reflecting for solar radiation. In both cases, the solar radiation
is able to reach the base of the cell, said base being the absorber. If
the collector is caused to follow the position of the sun, then it is also
possible to manage with thin honeycomb wall materials which are impervious
to solar radiation. The honeycomb walls then only have to stand parallel
to the incident radiation.
There is another condition which has to be set for the cell walls. They are
to be as thin as possible, so that the heat conduction in the cell walls
is small and does not make a considerable contribution to the heat losses
of the collector due to conduction in the material of the cell wall.
For the first time, cell structures have been used by Russian scientists
for solar energy collectors..sup.4 They used specially treated paper for
the manufacture of the honeycomb structures. A new impetus resulted from
the use of honeycomb structures by FRANCIA..sup.5 He used bunched glass
tubes for producing high temperatures. Foils of synthetic plastics
likewise initially seemed to be a very suitable material for the honeycomb
walls and experiments have also been carried out with these. PERROT et
al.sup.6 have experimented with honeycombs of synthetic plastic foils. The
results did not come up to expectations, since thin foils of synthetic
plastics are partially transparent for the long-wave infrared radiation.
BUCHBERG et al.sup.7 used paper as the wall material, said paper having
been vapor-coated with aluminum, so that the solar radiation was reflected
down to the absorber. The surface of the aluminum was coated with thick
lacquer coatings (transparent to solar radiation), the purpose of these
coatings being to provide for the long-wave infrared being absorbed in
them.
.sup.4 V. B. VEINBERG, Optics in Equipment for the Utilization of Solar
Energy, State Publishing House of Defense Ministry, Moscow (1959(,
(Translated by U.S. Dept. of Army Intelligence, Translation No. 44787, or
USAEC Translation AEC-tr-4471).
.sup.5 G. FRANCIA, Paper E/Conf. 35/5/71. U.N. Conf. on New Sources of
Energy, Rome (1961).
.sup.6 PERROT, Solar Energy, g (1967) Vol. 11, no. 1, pp. 34-40.
.sup.7 BUCHBERG, Solar Energy, Vol. 13, pp. 193-221, Pergamon Press, 1971.
If the prior experiments with solar energy collectors having honeycomb
structures for suppressing the heat losses are summarized, the conclusion
is reached that it would have been possible to construct very good
collectors in accordance with this principle, if only suitable materials
were available for the honeycomb structures.
The satisfactory materials are those which are transparent for solar
radiation. If such wall materials have an optically good surface, i.e., if
they only disperse the radiation to a very slight degree, and if the
material has low absorption power, then a very high percentage reaches the
bottom of the cells, that is to say, reaches the absorber. Wall materials
which are not transparent for solar radiation are basically less suitable,
since there are no simple coatings which reflect solar radiation free from
heat loss. A portion of the solar radiation accordingly does not reach the
absorber.
It is just as difficult and unsatisfactory as regards the power of
absorption for long-wave infrared, which is necessary so that the
radiation exchange is reduced.
If thin foils of plastic materials are used, then a considerable proportion
of the long-wave infrared radiation is allowed to pass through. It would
be possible to use thicker foils, in order to increase the absorption. It
is only a slight improvement which can be obtained in this way because of
the typical band structure of the infrared transmission spectra of organic
polymers. In spectral regions of high transmissions, it is necessary to
have foils of such great thickness in order to noticeably restrict the
transmission actually on the said foils, and the cost of the foil would
then be an obstacle.
For example, a Hostaphan foil with a thickness of 75 .mu.m still allows the
passage of about 20% of the radiation of a black body of 350.degree. C.
Copolymers having a composition which has been selected so that spectral
regions of great transmissivity of the one polymer are covered by the
absorption bands of the other polymer are of some assistance. Lacquered or
lined foils are likewise possible. An additional disadvantage is that the
plastic materials have to be exceptionally stable to various radiation
effects. As more different plastic materials contributing to the
fabrication of the foil, the more difficult it is to satisfy the stability
conditions in addition to the properties which have already been discussed
hereinbefore. A need therefore exists to eliminate or minimize the
aforementioned difficulties and disadvantages.
OBJECTS OF THE INVENTION
It is therefore a significant object of the present invention to provide a
fibrous wall material for cell structures of solar energy collectors,
comprising an arrangement of fibers, said fibers being substantially
transparent to solar radiation and substantially impervious for long-wave
infrared radiation, the fibers being arranged in layers in a parallel
relationship.
Another significant object of this invention is the provision of a solar
energy collecting unit wherein heat losses are substantially minimized.
A still further object of the present invention is the provision of a solar
energy collecting unit that can be simply and economically manufactured.
SUMMARY OF THE INVENTION
The foregoing and other objectives are achieved in accordance with the
present invention through the provision of fibrous wall structures useful
between an absorber and a transparent cover sheet, if used, in a solar
collecting unit. The material of the fibrous wall comprises an arrangement
of fibers wherein said fibers are substantially transparent to solar
radiation and substantially impervious to long-wave infrared radiation.
The fibers are further arranged in layers in a parallel relationship. The
invention also relates to the cells or honeycombs per se that are made
with said fibrous wall materials.
BRIEF DESCRIPTION OF THE DRAWINGS
Various other objects, features, and attendant advantages of the present
invention will be more fully appreciated as the same becomes better
understood from the following detailed description when considered in
connection with the accompanying drawings, and wherein:
FIG. 1a illustrates a wall structure in the form of a honeycomb having a
rectangular cross-section;
FIG. 1b illustrates a wall structure in the form of a honeycomb having a
hexagonal cross-section;
FIG. 2 is a perspective view of a solar energy collector of the present
invention;
FIG. 3 is a perspective view of another embodiment of the present invention
wherein supports are used at suitable intervals to support the frame over
which the fibrous wall material is applied; and
FIG. 4 is a perspective view of still another embodiment of the present
invention illustrating the use of alternately angular and straight frames.
DETAILED DESCRIPTION OF THE INVENTION
The fibers used in the practice of this invention may comprise any fibers
having the aforementioned properties. Particularly suitable fibers include
glass and mineral fibers. Suitable glass fibers include quartz glass,
E-glass, and glass compositions consisting of the following: (A) 12% boron
oxide, 80% silica, 5% alkali metal oxide, 3% alumina; and (b) 50% silica,
20% alumina, 9% boron oxide, and 13% alkaline earth oxide. Suitable
mineral fibers include conventional fibers such as rockwool.
The fibers may be arranged upon a supporting foil, preferably a foil that
is transparent to solar radiation. At least one layer of fibers may be
arranged in a parallel relationship with the fibers being bound to one
another by means of a plastic bonding material that is also transparent to
solar radiation. The fibers may preferably be wound or coiled over a frame
to provide a closed covering fiber wall, as illustrated in FIG. 2. When
glass fibers are used, they should preferably be arranged, one above the
other, so that the solar radiation has to penetrate a fiber thickness of
about 10 to 15 .mu.m, on the average. A double layer of fibers is
sufficient when using fibers having a thickness of 10 .mu.m. FIG. 2, of
the accompanying drawings shows, in perspective, a solar energy collector
illustrative of the present invention. Thin glass fibers 2 are arranged
parallel and in such a juxtaposition to one another that the fibers,
disposed in several layers, form a closed, covering wall 4. If a plane
(called "fiber wall plane", points E,F,G,H) common to all the axes of the
fibers 2 of the uppermost layer is drawn through the said axes, the axes
of the fibers should stand perpendicular to the section line of the fiber
wall plane with the absorber plane (line G-H).
The fiber wall plane is generally disposed perpendicular to the absorber
plane (points A,B,C,D), although it is possible, as will later be
explained, to deviate from this condition. In respect of a ray which lies
in an incidence plane (points J,K,L,M) which is, for example,
perpendicular to the fiber wall plane, it can be seen that the proportion
of the incident radiation which is reflected on the fibers is reflected
from the point of incidence P in many different directions; however, the
inclination of these different rays relative to the absorber plane remains
the same for each ray. Consequently, a radiation reflected several times
at different planes of the fiber wall safely reaches the base of the
honeycomb, i.e., the absorber 6. It can also be shown that the proportion
of the radiation which passes through the fibers always maintains its
inclination or slope relative to the absorber plane. If one departs from
the condition that the plane of the fiber wall is no longer perpendicular
to the absorber plane, then also in this case, most of the incident
radiation will reach the bottom of the honeycomb, provided the angle of
incidence between radiation and absorber plane remains smaller than the
angle between fiber wall plane and absorber plane (what is meant here is
the angle which is smaller than 90.degree.). It is understood that the
incident angle is, as usual, the angle between the normal to the absorber
plane and the incident beam or ray.
One possibility consists in arranging one or more layers of fibers in
parallel, on a supporting foil or frame, and adhering them to the foil by
means of a suitable binder. Neither the refractive index of the supporting
foil or frame, nor the refractive index of the adhesive, must be the same
as the refractive index of the fibers. The supporting foil only has to be
transparent for the solar radiation, and this condition also applies with
respect to the adhesive. Suitable polymeric binders include, for example,
polymethyl methacrylate.
According to another embodiment of the invention, a cellular, e.g.,
honeycomb structure useful in a solar energy collector is provided which
has been formed from the fibrous wall materials defined hereinbefore.
According to this embodiment of the invention, the fibrous wall materials
are in the form of a honeycomb wherein the cells thereof can have any one
of a number of cross-sections, e.g., rectangular, hexagonal, etc. In the
formation of these cell structures it is not necessary to use a supporting
foil although the same can be used if desired. When several layers of
fibers are arranged in a parallel relationship, said fibers being bonded
to one another by means of a plastic material, there is usually no need
for a supporting foil. Although the synthetic plastic bonding materials
used to bond the fibers together must be transparent for solar radiation,
it is not necessary for the refractive indices of the fibers and the
synthetic plastic bonding materials to be the same.
Another embodiment of the invention consists of fibers which are wound or
coiled over a frame. Suitable frames used for this purpose consist of
stamped sheet metal, tensioned wires or other materials forming a stable
frame wherein glass fibers are wound tightly thereover whereby a closed
fiber wall is formed. Honeycombs are then fabricated from these fiber
walls. This last mentioned embodiment is particularly suitable for
honeycomb structures or cells which are to be used at temperatures higher
than 200.degree. C. Most of the transparent plastics materials are only to
be used up to about 200.degree. C.
The fibers which are described herein do not in all cases have to be made
of glass provided they are transparent for the solar radiation and have a
strongly absorbing action in respect to long-wave infrared radiation.
Mineral fibers are likewise suitable and suitable mineral fibers include
rockwool.
If glass fibers are used, they should preferably be arranged one above the
other so that the solar radiation has, on the average, to penetrate a
fiber thickness of at least 10 to 15 .mu.m. a double layer of fibers is
sufficient when using fibers with a thickness of 10 .mu.m.
Glass fibers have the economic advantage that they are extremely efficient.
Quartz glass fibers are especially suitable for being used at high
temperature.
According to another embodiment of the invention, a solar energy collector
is provided comprising a solar radiation absorber and one or more cell
structures as defined in the preceding paragraph. Generally the axes of
the fibers are perpendicular to the section line between the fiber wall
plane and the absorber plane.
The invention will be better understood by making reference to the
following examples which illustrate preferred embodiments of the
invention:
EXAMPLE 1
Wall Material of Fibers Bonded to One Another
Glass fibers (E-glass) with a thickness of 10 .mu.m are wound tightly and
uniformly over a rod-type drum, so that two to four layers of fibers
assume a parallel position one above the other. These fibers are bonded to
one another with polymethyl methacrylate (PMMA), by immersing the drum
briefly in dissolved PMMA, or by PMMA being applied to the fibers.
Chloroform is recommended as solvent. After the curing of the PMMA, the
fiber foil is cut off from the drum in a suitable width.
EXAMPLE 2
Wall Material of Fibers Wound over a Frame
Glass fibers (E-glass) having a thickness of 10 .mu.m are wound
sufficiently tight, over stamped-out sheet metal frames resulting in two
to four layers of fibers being arranged parallel one above the other. The
sheet metal frame 10 is made of thin (0.1 to 0.3 mm) sheet metal. If
necessary, supports 8 can be included in the frame at suitable intervals
as shown in perspective in FIG. 3 of the accompanying drawings. A great
improvement in the stability of the frame and simultaneously a suitable
honeycomb cross-section is obtained by the frame around which the fibers 2
are wound being bent over at regular intervals. FIG. 4 of the accompanying
drawings shows in perspective a honeycomb structure which is composed of
alternately angular and straight frames, for example, 12 and 14.
It is noted that the frames that are used in the present invention to form
the fibrous walls, can be any hard, solid material, e.g., metal, plastic,
wood and the like.
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