U.S. patent application number 10/029493 was filed with the patent office on 2002-06-20 for solar collector element.
Invention is credited to Ganz, Klaus, Reichert, Werner.
Application Number | 20020073988 10/029493 |
Document ID | / |
Family ID | 7950385 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020073988 |
Kind Code |
A1 |
Reichert, Werner ; et
al. |
June 20, 2002 |
Solar collector element
Abstract
The invention relates to a solar collector element having an
absorber part and a tube for a heat transfer liquid connected
thereto on a first side. The absorber part consisting of a
composite material having a metallic substrate and an optically
active coating on a second side of the substrate. The coating is a
multilayer system having three layers. The top layer is a
dielectric layer, preferably an oxide, fluoride or nitride layer of
chemical composition MeO.sub.z, MeF.sub.r, MeN.sub.s, having a
refractive index n<1.8. The middle layer is a chromium oxide
layer of chemical composition CrO.sub.x. The bottom layer is gold,
silver, copper, chromium, aluminium and/or molybdenum. The indices
x, z, r and s indicate a stoichiometric or non-stoichiometric ratio
in the oxides, fluorides or nitrides.
Inventors: |
Reichert, Werner;
(Wuppertal, DE) ; Ganz, Klaus; (Wuppertal,
DE) |
Correspondence
Address: |
Steven L. Oberholzer
Brinks Hofer Gilson & Lione
P.O. Box 10396
Chicago
IL
60610
US
|
Family ID: |
7950385 |
Appl. No.: |
10/029493 |
Filed: |
December 20, 2001 |
Current U.S.
Class: |
126/676 ;
126/677; 126/907; 126/908 |
Current CPC
Class: |
Y02E 10/44 20130101;
F24S 70/25 20180501; F24S 10/75 20180501; F24S 70/30 20180501; F24S
70/225 20180501; B23K 26/22 20130101 |
Class at
Publication: |
126/676 ;
126/677; 126/907; 126/908 |
International
Class: |
F24J 002/48 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2000 |
DE |
200 21 644.9 |
Claims
1. Solar collector element comprising an absorber part and a tube
adapted to contain a heat-transfer liquid, the tube being connected
to the absorber part on a first side, the absorber part being a
composite material having a metallic substrate and having an
optically active coating on the substrate on a second side (A), the
coating further comprising a multilayer system having three layers,
the top layer being a dielectric layer having a refractive index
n<1.8, the middle layer being a chromium oxide layer of chemical
composition CrO.sub.x, and the bottom layer being of a material
selected from the group consisting of gold, silver, copper,
chromium, aluminium and molybdenum, the index x indicates a
stoichiometric or non-stoichiometric ratio.
2. Solar collector element according to claim 1, wherein the top
layer is a silicon oxide layer of chemical composition SiOy, the
index y indicating a stoichiometric or non-stoichiometric
ratio.
3. Solar collector element according to claim 1 wherein the top
layer is of a chemical composition selected from the group
consisting of MeO.sub.z, MeF.sub.r and MeN.sub.s the indices z, r
and s indicating a stoichiometric and non-stoichiometric ratio.
4. Solar collector element according to claim 1 wherein an
intermediate layer is applied to the substrate beneath the
multilayer system.
5. Solar collector element according to claim 1 wherein a lower
layer is applied to the substrate on the first side thereof.
6. Solar collector element according to claim 1 wherein the
substrate is formed of aluminium.
7. Solar collector element according to claim 6 wherein the
aluminium is more than 99.0% pure.
8. Solar collector element according to claim 4 wherein the
intermediate layer is formed of anodically oxidized aluminium.
9. Solar collector element according to claim 4 wherein the
intermediate layer is formed of electrolytically brightened and
anodically oxidized aluminium.
10. Solar collector element according to claim 5 wherein the lower
layer is formed of anodically oxidized aluminium.
11. Solar collector element according to claim 5 wherein the lower
layer is formed of electrolytically brightened and anodically
oxidized aluminium.
12. Solar collector element according to claim 1 wherein the
substrate has a rolled structure of grooves which run substantially
parallel to one another in a preferred direction.
13. Solar collector element according to claim 1 wherein the
substrate is formed of copper.
14. Solar collector element according to claim 1 wherein the
stoichiometric or non-stoichiometric ratio x lies in the range
0<.times.<3.
15. Solar collector element according to claim 2 wherein the
stoichiometric or non-stoichiometric ratio y lies in the range
1.ltoreq.y.ltoreq.2.
16. Solar collector element according to claim 1 wherein the bottom
layer includes a plurality of partial layers, arranged one above
the other and be formed of at least one material selected from the
group of gold, silver, copper, chromium, aluminium and
molybdenum.
17. Solar collector element according to claim 1 wherein at least
one of the top and middle layers are sputtered layers.
18. Solar collector element according to claim 17 wherein the
layers are produced by reactive sputtering.
19. Solar collector element according to claim 1 wherein at least
one of the top and middle layers are produced by vaporization.
20. Solar collector element according to claim 19 wherein
vaporization is by electron bombardment.
21. Solar collector element according to claim 19 wherein the
vaporization is by thermal sources.
22. Solar collector element according to claim 1 wherein at least
one of the upper and middle layers are CVD layers.
23. Solar collector element according to claim 1 wherein at least
one of the upper and middle layers are PECVD layers.
24. Solar collector element according to claim 1 wherein the bottom
layer is a sputtered layer.
25. Solar collector element according to claim 1 wherein the bottom
layer is a layer produced by vaporization.
26. Solar collector element according to claim 25 wherein
vaporization is by electron bombardment.
27. Solar collector element according to claim 25 wherein
vaporization is from thermal sources.
28. Solar collector element according to claim 1 wherein the
multilayer system is applied in vacuum order in a continuous
process.
29. Solar collector element according to claim 1 wherein the top
layer has a thickness in the range of 3 nm to about 500 nm.
30. Solar collector element according to claim 1 wherein the middle
layer has a thickness in the range of 10 nm to about 1 .mu.m.
31. Solar collector element according to claim 1 wherein the bottom
layer of the optical multilayer system has a thickness (D.sub.6) of
at least 3 nm and at most approximately 500 nm.
32. Solar collector element according to claims 1 wherein a total
light reflectivity on second side is less than 5%.
33. Solar collector element according to claim 1 wherein a total
light reflectivity on the second side under a thermal load of
430.degree. C./100 hours undergoes changes of less than 7%.
34. Solar collector element according to claim 33 wherein the
change is less than 4%.
35. Solar collector element according to claim 1 wherein the
absorber part is of plate-like form and has a thickness in the
range of 0.1 to about 1.5 mm.
36. Solar collector element according to claim 35 wherein the
thickness is in the range of about 0.2 to about 0.8 mm.
37. Solar collector element according to claim 1 wherein the tube
is formed of copper.
38. Solar collector element according to claim 1 wherein the
absorber part and the tube are connected to one another by means of
a material-to-material laser welded bond.
39. Solar collector element according to claim 38 wherein the bond
is formed by a pulse welding process.
40. Solar collector element according to claim 38 wherein the bond
between the absorber part and the tube is made up of only of the
respective materials of the absorber part and of the tube.
41. Solar collector element according to claim 24 wherein the
absorber part and the tube with the absorber part having a
substrate made from aluminium and the tube is formed of copper, is
formed by a series of molten balls which have solidified on the
absorber part and predominantly made up of aluminium and by
diffusion of the aluminium into the copper of the tube.
42. Solar collector element according to claim 24 wherein the tube
and the absorber part are joined where they are in abutment with
one another by weld seams running on both sides of the tube and are
formed from weld spots which are spaced apart from one another.
43. Solar collector element according to claim 41 wherein the tube
and the absorber part, with the absorber part having a thickness in
the range of about 0.3 to about 0.8 mm and a diameter of the molten
balls in the range of about 0.2 to about 3.2 mm spaced at a
distance in the range of about 0.5 to about 2.5 mm between centers
of the molten balls.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a solar collector element
having an absorber part and having a tube for a heat-transfer
liquid. The absorber part is a composite material having a metallic
substrate with an optically active coating on a second side of the
absorber part opposite the tube.
[0002] It is known to use solar collectors to obtain energy from
solar radiation. The solar radiation is converted into heat at an
absorber part, which is in the form of a plate, for example, of a
solar collector and heats a heat-transfer liquid contained in the
collector. For the heat-transfer liquid, there is a circuit system
composed of tubes that enables the heat which has been taken up to
be released again to a consumer, such as for example to a heat
exchanger, in which service water can be heated, or to heat a
swimming pool.
[0003] In general, when radiation impinges on an object--as is the
case with the coated surface of an absorber part--it is split into
a reflected fraction, an absorbed fraction and a transmitted
fraction, which are determined by the reflectivity (reflectance),
the absorptivity (absorptance) and the transmissivity
(transmittance) of the object. Reflectance, absorptance and
transmittance are optical properties which, depending on the
wavelength of incident radiation (e.g. in the ultraviolet region,
in the region of visible light, in the infrared region and in the
region of thermal radiation) can adopt different values for the
same material. Kirchhoff's law, according to which the
absorptivity, in each case at a defined temperature and wavelength,
has a constant ratio to the emittance, is known to apply to the
absorptance. Therefore, Wien's displacement law and Planck's
radiation law as well as the Stefan-Boltzmann law are of importance
for the absorptance, describing defined relationships between
radiation intensity, spectral distribution density, wavelength and
temperature of a black body. Calculations should take account of
the fact that the black body per se does not exist, and real
substances each deviate in a characteristic way from the ideal
distribution. To ensure highly effective utilization of energy,
absorber parts are required to have a maximum absorptivity in the
solar wavelength region (approximately 300 to approximately 2500
nm) and a maximum reflectivity in the thermal radiation region
(above approximately 2500 nm).
[0004] Absorbers for flat collectors, in which solar collector
elements of the type described above with coated absorber parts
that satisfy this demand for selective absorption to a high level,
are known under the name Tinox. In these collectors, the material
of the absorber parts comprises a copper strip substrate to which a
layer of titanium oxynitride has been applied, followed by a
covering layer of silicon dioxide. The tube which is connected to
an absorber part likewise consists of copper and is soldered to the
absorber part.
[0005] In connection with solar collectors a distinction is drawn
between low-temperature collectors, with operating temperatures of
up to 100.degree. C., and high-temperature collectors, with
operating temperatures of over 100.degree. C. In the case of tower
installations, which are used to provide process heat, the absorber
temperature may be up to 1200.degree. C. The so-called steady
temperature, which is to be understood as meaning the theoretically
possible maximum temperature of use of a collector at which the
material is in thermal equilibrium with the environment, is often
referred to as a characteristic variable for a solar collector.
Steady temperatures in the low-temperature range are characteristic
of the Tinox absorbers described. In higher temperature ranges, for
the known solar collector element with the Tinox coating or with
other coatings which are known to be used, such as black paints or
pigmented plastics, there is a risk of decomposition of the layer,
of gases being evolved, but also of the capacity of the collector
element falling at least for a relatively short time, for example
on account of bleaching of the black layer.
[0006] With regard to the joining of the absorber part and tube, it
should be noted that this jointing only has to ensure the required
strength, but also has to provide a sufficiently high heat
transfer. In connection therewith, it should be ensured that, in
the event of any desired changes in the absorber part/tube
combination of materials (no longer Cu/Cu), problems do not arise
with regard to finding a suitable joining technique.
[0007] The present invention is based on the object of providing a
solar collector element of the type described in the introduction
which, on the one hand, leads to a high light absorption and a high
reflectivity in the solar radiation region and, on the other hand,
which has improved use characteristics in particular under high
thermal load operating conditions, and has a longer service life,
in combination with a production method that involves minimum
possible capital outlay. Furthermore, the invention is also
intended to allow an optimum solution to the problem with regard to
ensuring a strong mechanical joint and good heat transfer between
the absorber part and the tube. Finally, the solar collector
element is to be distinguished by the possibility of steady
temperatures within the range of use of high-temperature collectors
and also by high long-term chemical stability.
SUMMARY OF THE INVENTION
[0008] According to the invention, this is achieved by the fact
that the coating comprises a multilayer system which is composed of
three layers, of which the top layer is a dielectric layer,
preferably an oxide, fluoride or nitride layer of chemical
composition MeO.sub.z, MeF.sub.r, MeN.sub.s, having a refractive
index n<1.8, and of which the middle layer is a chromium oxide
layer of chemical composition CrO.sub.x, and of which the bottom
layer consists of gold, silver, copper, chromium, aluminium and/or
molybdenum. The indices x, z, r and s indicating a stoichiometric
or non-stoichiometric ratio in the oxides, fluorides or
nitrides.
[0009] The optical multilayer system which is present according to
the invention can firstly be applied advantageously, since there is
no need for environmentally hazardous, in some cases toxic, salt
solutions during production. For example, the metallic layer of the
optical multilayer system may be a sputtered layer or a layer which
is produced by vaporization, in particular by electron bombardment
or from thermal sources. The two upper layers of the optical
multilayer system may likewise be sputtered layers, in particular
layers produced by reactive sputtering, CVD or PECVD layers or
layers produced by vaporization, in particular by electron
bombardment or from thermal sources, so that the entire optical
multilayer system comprises layers which are applied in vacuum
order, in particular in a continuous process.
[0010] The top layer may alternatively be a silicon oxide layer of
chemical composition SiOy, where the index y once again indicates a
stoichiometric or non-stoichiometric ratio in the oxide
composition.
[0011] In addition to having a high long-term thermal and chemical
stability, the solar collector element according to the invention
is also distinguished, on account of the ease of processing, in
particular deforming, the composite material from which the
absorber part is produced. This is primarily achieved on account of
the metallic substrate, which may be made from copper or preferably
from aluminium, by a production method which involves little
complexity and by a high thermal conductivity. The latter property
is particularly important since it allows rapid, highly efficient
transfer of the heat taken up as a result of the light absorption
to the heat-transfer liquid.
[0012] The above processes for applying the layers of the system
also advantageously enable the chemical composition MeO.sub.z,
MeF.sub.r, MeN.sub.s of the top layer and the chemical composition
CrO.sub.x of the chromium oxide layer, with regard to the indices
x, y, z, r and s, to be not only set at defined, discrete values
but also allows a stoichiometric or non-stoichiometric ratio
between the oxidized substance and the oxygen to be varied
continuously within defined limits. In this way it is possible to
specifically set, by way of example, the refractive index of the
reflection-reducing top layer (which is also responsible for
increasing the mechanical load-bearing capacity (DIN 58196, part
5)) and the absorptivity of the chromium oxide layer (the
absorptance decreasing as the value of the index x rises).
[0013] According to the invention, it is in this way possible to
set a total light reflectivity, determined in accordance with DIN
5036, part 3, on the side of the optical multilayer system to a
preferred level of less than 5%. In addition to a high resistance
to ageing, it is also possible to ensure a high thermal stability,
in such a manner that under a thermal load of 430.degree. C./100
hours, only changes of less than 7%, and preferably of less than
4%, in the reflectivity occur. Moreover, in the event of a thermal
load of this nature, there is advantageously also no evolution of
gases.
[0014] The composite material which is used for the absorber part
according to the invention therefore, on account of its synergistic
combination of properties
[0015] of the substrate layer, for example its excellent
deformability, by means of which it withstands stresses produced in
the production process of the solar collector element according to
the invention during the shaping processes which are to be
performed without problems, for example its high thermal
conductivity and--in particular in the case of aluminium as
substrate material--the capacity for a surface patterning which in
the light wavelength region additionally promotes adsorption and is
then followed by the other layers in relief, and moreover with a
reflectance in the solar radiation region which reinforces the
action of the metallic layer of the optical three-layer system;
[0016] of the metallic layer which, on account of its constituents,
which have a high reflectivity and therefore a low emission in the
thermal radiation region, takes account of the fact that, according
to the Lambert-Bouguer law, the radiation characteristic is
absorbed exponentially as the penetration depth grows, and for most
inorganic substances is available as a store or thermal energy
which can be passed on at even a very low depth (less than
approximately 1 .mu.m);
[0017] of the chromium oxide layer, with its high selectivity of
the absorptivity (peak values over 90% in the wavelength region
from approximately 300 to 2500 nm, minimum values below 15% in the
wavelength region>approx. 2500 nm) and its capacity for
modification (index x) which has already been explained, and
[0018] of the top, in particular silicon oxide, layer, the
advantages of which have to some extent already been pointed out
above and which, in addition to its antireflective action, also has
a high transmittance and, as a result, increases the proportion of
the radiation values in the solar region which can be absorbed by
the chromium oxide layer;
[0019] is eminently suitable for the production of the solar
collector element according to the invention. Therefore, by using
the solar collector element according to the invention, not only is
it possible to produce low-temperature collectors with an operating
temperature of up to 100.degree. C., but also it is possible to
produce high-temperature collectors, in which steady temperatures
of over 250.degree. C. are possible.
[0020] Beneath the optical multilayer system, it is also possible
to provide an intermediate layer on the substrate, the intermediate
layer being responsible firstly for mechanical and
corrosion-inhibiting protection for the substrate and secondly for
high adhesion for the optical multilayer system. A lower layer may
be applied to the substrate--likewise as a protection layer--on the
side which is remote or opposite from the side to which the optical
multilayer system is applied. In the case of an aluminium
substrate, both layers may consist of aluminium oxide, which can be
produced from the anodically oxidized or electrolytically
brightened and anodically oxidized substrate material. In addition,
a further layer, with a reflection-increasing action, can be
applied to the substrate or to the lower layer.
[0021] Preferably, in the case of an aluminium substrate material
of the absorber part and a copper tube, these two parts can
advantageously be joined to one another (even if a layer consisting
of aluminium oxide is present on the substrate) by laser welding.
This leads to a material-to-material bond as a result of melted and
resolidified aluminium and as a result of migration of the
aluminium into the copper. By way of example, radiation from a
CO.sub.2 or Nd-YAG laser with sufficient power can be used for the
welding.
[0022] Irrespective of the combination of materials used for the
absorber part/tube, the use of laser welding has advantages over
soldering. One advantage is that there are no unnecessary metallic
interfaces (no solder). Another advantage is that it is possible to
achieve a higher mechanical strength of the joint and, therefore, a
greater resistance to vibrations and impacts. A further advantage
is that the permissible operating temperature (steady temperature)
of the absorber part according to the invention is increased. Still
another advantage is that a greater operational reliability is
ensured.
[0023] In particular, the tube and the absorber part may be joined
where they are in abutment with one another by spot-welded seams
which run on both sides of the tube and are produced by a pulse
welding process. When determining the laser power and pulse
frequency, it should be noted that the weld spot dimensions are
primarily dependent on the thermal conductivity, with surface
temperature, irradiation time, thickness of the absorber part and
type of material representing factors which influence one another.
There is a proportional relationship between the fusion depth and
the mean laser power.
[0024] Further advantageous embodiments of the invention are given
in the subclaims and in the detailed description which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The invention is explained in more detail on the basis of an
exemplary embodiment which is illustrated by the appended drawing,
in which:
[0026] FIG. 1 shows an outline sectional illustration through an
absorber part of a solar collector element according to the
invention;
[0027] FIG. 2 shows a perspective view of an area of an embodiment
of a solar collector element according to the invention; and
[0028] FIG. 3 shows a plan view of an embodiment of a solar
collector element according to the invention with a meandering tube
path.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Throughout the various figures of the drawing, identical
parts are always provided with identical reference symbols and are
therefore generally also in each case only described once.
[0030] The absorber part (reference symbol 10 in FIG. 2) of the
solar collector element (reference symbol E in FIG. 2) according to
the invention consists of a composite material with a highly
selective absorptivity and reflectivity in the solar wavelength
region and in the thermal radiation region. This composite material
for its part comprises a strip-like substrate 1, which in
particular can be deformed and consists of aluminium, an
intermediate layer 2 which is applied to the substrate 1 on a side
A, and an optically active multilayer system 3 which is applied to
the intermediate layer 2.
[0031] The substrate 1 may preferably have an in particular regular
rolled structure of grooves which run substantially parallel to one
another in a preferred direction. A structure of this type, if
these grooves are oriented parallel to the north/south direction,
enables the absorptance of a solar collector element E, according
to the invention, to attain a level which is as far as possible
independent of the particular angle of the sun, which changes over
the course of the day.
[0032] A total light reflectivity, determined according to DIN
5036, part 3, on side A of the optical multilayer system 3 may
preferably be less than 5%.
[0033] The composite material may preferably be processed in the
form of a coil with a width of up to 1600 mm, preferably of 1250
mm, and with a thickness D of approximately 0.1 to 1.5 mm,
preferably of approximately 0.2 to 0.8 mm, it being possible to
produce the solar collector element E according to the invention
from this coil in a simple manner by stamping out a plate-like
absorber part 10 and joining it to a tube (reference symbol 11 in
FIG. 2). The substrate 1 of the composite material may preferably
have a thickness D.sub.1 of approximately 0.1 to 0.7 mm.
[0034] The aluminium of the substrate 1 may in particular be more
than 99.0% pure, which promotes a high thermal conductivity.
[0035] (The intermediate layer 2 consists of anodically oxidized or
electrolytically brightened and anodically oxidized aluminium,
which is applied to the substrate material.
[0036] The multilayer system 3 comprises three individual layers 4,
5, 6, an upper and middle layers 4, 5 being oxide layers and a
bottom layer 6 being a metallic layer applied to the intermediate
layer 2. The top layer 4 of the optical multilayer system 3 is,
preferably, a silicon oxide layer of chemical composition
SiO.sub.y. The middle layer 5 is a chromium oxide layer of chemical
composition CrO.sub.x, and the bottom layer 6 consists of gold,
silver, copper, chromium, aluminium and/or molybdenum.
[0037] The indices x, y indicate a stoichiometric or
non-stoichiometric ratio of the oxidized substance to the oxygen in
the oxides. The stoichiometric or non-stoichiometric ratio x may
preferably lie in the range 0<.times.<3, while the
stoichiometric or non-stoichiometric ratio y may adopt values in
the range 1.ltoreq.y.ltoreq.2.
[0038] The fact that the two upper layers 4, 5 of the optical
multilayer system 3 may be sputtered layers, in particular layers
produced by reactive sputtering, CVD or PECVD layers or layers
produced by vaporization, in particular by electron bombardment or
from thermal sources, means that it is possible to adjust the
ratios x, y continuously (i.e. to set them to non-stoichiometric
values of the indices), with the result that the layer properties
can in each case be varied.
[0039] The top layer 4 of the optical multilayer system 3 may
advantageously have a thickness D.sub.4 of more than 3 nm. At this
thickness D.sub.4, the layer is already sufficiently efficient, yet
the outlay on time, material and energy is low. An upper limit for
the layer thickness D.sub.4, in view of these aspects, is
approximately 500 nm. An optimum value for the middle layer 5 of
the optical multilayer system 3, in view of the abovementioned
aspects, is a minimum thickness D.sub.5 of more than 10 nm and a
maximum thickness D.sub.5 of approximately 1 .mu.m. The
corresponding value for the bottom layer 6 is a thickness D.sub.6
of at least 3 nm, at most approximately 500 nm.
[0040] With a view to achieving high efficiency, the bottom layer 6
of the optical multilayer system 3 should preferably be more than
99.5% pure. As has already been mentioned, the layer may be a
sputtered layer or a layer which is produced by vaporization, in
particular by electron bombardment or from thermal sources, so that
the entire optical multilayer system 3 advantageously comprises
layers 4, 5, 6 which are applied in vacuum order in a continuous
process.
[0041] A lower layer 7 which--like the intermediate layer
2--consists of anodically oxidized or electrolytically brightened
and anodically oxidized aluminium, is applied to that side B of the
strip-like substrate 1 which is remote from the optical multilayer
system 3. The intermediate layer 2 and the lower layer 7 may
advantageously be produced simultaneously by wet-chemical means, in
which case the pores in the aluminium oxide layer can be as far as
possible closed off by hot-sealing during the final phase of the
wet-chemical process sequence, resulting in a surface with
long-term stability. Therefore, the lower layer 7--like the
intermediate layer 2--offers mechanical and corrosion-inhibiting
protection to the substrate 1.
[0042] According to the invention it is possible, in particular,
for the layer structure to be assembled in such a manner that the
total light reflectivity, determined in accordance with DIN 5036,
part 3, on side A of the optical multilayer system 3, under a
thermal load of 430.degree. C./100 hours, undergoes changes of less
than 7%, preferably of less than 4%.
[0043] FIG. 2 illustrates the overall structure of a solar
collector element E according to the invention. The drawing
diagrammatically depicts the absorber part 10 and the tube 11 (for
a heat-transfer liquid) as parts of the solar collector element E.
The absorber part 10 consists of the composite material having the
substrate 1 consisting of aluminium and the multilayer system 3
built up from three layers 4, 5, 6, as has been explained above.
The absorber part 10, which can be produced at low cost and in an
environmentally friendly manner, results in high light absorption
and dissipation of heat to the tube 11, while, under collector
operating conditions which involve high thermal loads, it is
possible to ensure a comparably long service life.
[0044] The nature of the joint between the absorber part 10 and the
tube 11 (which consist in particular of copper) is produced by
means of a laser welding process, in particular in the form of a
pulse welding process, also contributes to the latter effect. Laser
welding is a fusion welding process, i.e. the parts which are to be
joined are melted under the action of the laser radiation. A
particular feature is the high power density and, when using pulse
welding, the rapid cooling associated with the short duration of
action. Since the laser welding of the absorber part 10 to the tube
11 is preferably carried out without filler, the
material-to-material bond which is formed between the two parts
which are to be joined consists only of the respective materials of
the absorber part 10 and of the tube 11; on account of the lower
melting point of aluminium, drop-shaped solidified small molten
balls 12 predominantly comprising aluminium are formed on the
absorber part 10, and the aluminium has diffused into the copper of
the tube 11. The small molten balls 12 are responsible for bridging
any gap or air cushion which may be present between absorber part
10 and tube 11. To produce an optimum joint, the power density of
the laser during welding, taking account of the criteria listed
above, should not exceed 10.sup.7 W/cm.sup.2, preferably 10.sup.6
W/cm.sup.2. The total energy for a weld spot should be active for a
time of up to approximately 10 ms, preferably distributed over the
course of time. As well as the criteria which have already been
listed, the actual spatial and temporal intensity distribution at
the location of action should also be taken into account (spiking,
hot spots).
[0045] In particular, the tube 11 and the absorber part 10 may--as
illustrated in FIG. 2--be joined where they are in contact with one
another by weld seams which run on both sides of the tube 11 and
are formed from welds spots (small molten balls 12) which are
spaced apart from one another (distance a) and are in particular
arranged at regular intervals.
[0046] Since, when a solar collector is operating, the heat
transfer from the absorber part 10 to the tube 11 takes place
predominantly at the weld spots, the size of the small molten balls
12 and the distance a between the small molten balls 12 are the
decisive factors in determining the efficiency of the collector. On
the other hand, the heat resistance of the absorber part 10, in its
plane of extent, limits the efficiency of the collector. This heat
resistance of the absorber part 10 is determined substantially by
the thermal conductivity of the composite material, primarily that
of the substrate 1, on the one hand, and by the thickness D of the
absorber part 10, on the other hand. The optimum distance a between
the small molten balls 12, for a predetermined composite material
of the absorber part 10 and a fixed size (diameter d) of the small
molten balls 12, therefore depends on the thickness D of the
absorber part. In the case of a substrate 1 made from aluminium, a
thickness D of the absorber part of approximately 0.3 to 0.8 mm,
and a diameter d of the small molten balls 12 of approximately 0.2
to 3.2 mm, this optimum distance a (center-to-center distance of
the small molten balls 12) is approximately 0.5 to 2.5 mm. The
greater the thickness D of the absorber part 10, the shorter the
distance a between the weld spots has to be.
[0047] The present invention is not restricted to the exemplary
embodiment which has been described, but rather encompasses all
means and measures which achieve the same effect within the context
of the invention. For example, it is also possible for the bottom
layer 6 of the optical multilayer system 3 to comprise a plurality
of partial layers of gold, silver, copper, chromium, aluminium
and/or molybdenum arranged above one another.
[0048] As has already been mentioned, the top layer may
alternatively also consist of fluorides or nitrides. As is known,
copper is also eminently suitable as the substrate material,
although aluminium, for approximately the same heat transfer
properties, achieves a higher strength without it being necessary
to provide beads.
[0049] With aluminium strip as substrate 1, there is a wide range
of different rolled surfaces available, in particular surfaces with
a grooved structure, which, when used as absorber composite
material, advantageously minimize and homogenise the extent to
which the absorptance is dependent on the angle of the sun, given a
suitable orientation.
[0050] Furthermore, the person skilled in the art can supplement
the invention by means of additional advantageous measures without
departing from the scope of the invention. For example, the tube 11
may in particular be laid in straight form or, as illustrated in
FIG. 3, in meandering form on the absorber part 10. If the tube is
laid in meandering form, the welding can be restricted to straight
sections I of tube, while curved sections K of tube are not
welded.
[0051] Furthermore, the invention is not restricted to the
combination of features defined in claim 1, but rather may also be
defined by any other desired combination of specific features of
all the individual features disclosed. This means that in principle
virtually any individual feature of claim 1 can be omitted or
replaced by at least one individual feature disclosed elsewhere in
the application. In this respect, claim 1 is only to be understood
as an initial attempt at putting an invention into words.
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