Note: Descriptions are shown in the official language in which they were submitted.
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w ABSORBER FOR A THERMAL COLLECTOR OF A SOLAR SYSTEM AND
METHOD FOR THE PRODUCTION THEREOF
The present invention relates to an absorber for a thermal
collector of a solar installation having an absorber wing
for light and heat conversion and a pipe system for a heat
transfer medium, the pipe system being positioned between
two metal sheets, which lie one on top of the other,
forming the absorber wing, the shape of the pipe system
being introduced into at least one of the metal sheets and
the metal sheets being bonded to one another.
Absorbers are components of solar installations. Solar
installations principally comprise a solar receiving
surface, generally referred to as a collector, the solar
loop, and the heat accumulator. The collectors are
typically mounted on the roof of a house and convert the
incident solar radiation into heat. Pipelines, in which a
heat transfer medium, such as a water-glycol mixture, is
pumped in a loop, connect the collector to the heat
accumulator. The pumps are automatically switched into the
solar loop via a controller when a temperature sensor
signals that the temperature at the collector is higher
than that in the heat accumulator. The heat of the heat
transfer agent is dissipated to the accumulator water in
the heat accumulator.
In the private field of use, thermal collectors are used in
particular. These are collectors which absorb the incident
solar radiation and convert it directly into heat. The main
component of every thermal collector is the absorber. This
is a metallic, dark-colored plate which is also partially
made of plastic. Since the solar radiation is not
transmitted by the absorber and is also hardly reflected,
it is largely converted into heat which drains off via the
pipe system connected to the absorber. The heat transfer
agent is located in the pipe system. The absorber, with the
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v associated pipe system, is located in a weatherproof
housing having a glass cover. The air layer enclosed by the
glass cover of the housing and the absorber is used as a
transparent thermal insulation in the direction of the
incident sola r radiation. An insulating layer attached
below the absorber prevents heat losses via the housing
floor. The pipe system positioned below the absorber plate
is typically made of a meandering, pressure-resistant
copper pipe, which is connected at each end to a collection
line in order to connect multiple collectors to one
another.
The heat transfer between the absorber plate and the
absorber pipe occurs via a linear weld seam between the top
of the absorber pipe and the absorber plate. The heat
transfer via the linear weld seam is not optimal. For this
reason, Solvis GmbH has developed a wide soldered
connection between the absorber plate and the absorber pipe
that is to cause improved heat transfer. The soldered
connection extends over a graduated circle on the top of
the absorber pipe; the soldered connection is produced by
filling up the gusset between the pipe mantle and the
bottom of the absorber with solder.
In addition to the higher costs of this connection, the
heat transfer between absorber plate and absorber pipe is
still not optimal. In addition, the manufacturing costs of
typical collectors, which are high anyway, are
disadvantageous, which is largely caused by the complex
absorbers.
An absorber according to the species of a solar
installation having a pipe system for a heat transfer
medium, in which the pipe system is positioned between two
metal sheets lying one on top of another in that form the
absorber, the shape of the pipe system being introduced
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-~ into one of the metal sheets and the metal sheets being
bonded to one another, is known from US 4,089,324.
DE 195 46 100 A1 and US 4,299,202 disclose absorbers for a
solar collector whose metal sheets lying one on top of
another, which form the absorber, are glued to one another
at the contact surfaces.
On the basis of this related art, the present invention is
based on the object of providing an absorber having reduced
manufacturing costs and improved dimensional accuracy while
simultaneously having a reduced reject rate, in which
strength losses of the bond between the metal sheets of the
absorber occurring at high temperatures are compensated
for. Furthermore, the present invention is based on the
object of suggesting a method for manufacturing the
improved absorber.
This object is achieved in an absorber of the type cited at
the beginning in that the metal sheets are bonded to one
another using an adhesive and additionally in a formfitting
way using clinching.
Because the shape of the pipe system is introduced into at
least one, preferably both metal sheets of the absorber
wing, a significantly larger transfer area is available for
the incident heat.
The metal sheets are bonded to one another using an
adhesive and additionally in a formfitting way.
The joining of the metal sheets through an adhesive
significantly reduces the manufacturing costs, since
soldering or welding work for manufacturing the absorber
may be completely dispensed with.
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The joining of the sheets by an adhesive not only results
in a significant cost reduction, but rather additionally
improves the dimensional accuracy of the solar collector
while simultaneously reducing the reject rate. The energy
use is significantly reduced in gluing in relation to the
current bonding technologies.
An adhesive from the group of silicone, epoxide, or phenol
resin adhesives is preferably used as the adhesive.
Thermosetting adhesives based on modified epoxide resins
particularly have a high long-term resistance to changing
temperatures. Furthermore, these adhesives have favorable
processing conditions and strength and resistance
properties for metal/metal bonds.
As already noted at the beginning, there is no circulation
of the heat transfer medium in the Bola r loop under
specific operating circumstances. In this case,
temperatures of 200 °C - 220 °C may occur in the absorber
with appropriate solar radiation. In order to be able to
compensate for strength losses of the adhesive bond
occurring at these temperatures, the two metal sheets of
the absorber wing are additionally bonded to one another in
a formfitting way. Multiple Tox points between the metal
sheets have been shown to be the preferred additional bond.
The Tox points are produced through clinching of the two
metal sheets. This connection may be automated especially
easily and therefore manufactured cost-effectively.
Vaporization of the heat transfer medium is particularly
connected to the standstill of the normal loop. This
results in elevated corrosion or oxidation on the interior
walls of the pipe system if the typical water-glycol
mixture is used in particular.
In order to ensure sufficient stability of the absorber,
the metal sheets are provided at least in the region
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forming the inner surfaces of the 'pipe system with a
coating that inhibits corrosion and/or oxidation. However,
the metal sheets are preferably coated completely and on
both sides. Aluminum sheets are preferably anodized, while
sheet steel is particularly provided with a copper or
plastic coating. The coating ensures the desired longevity
of the solar collectors.
A roll bond method for manufacturing evaporator plates is
known from a brochure of Showa Aluminum Corporation, Osaka,
Japan 1993, in which metal sheets lying one on top of
another are welded to one another through hot rolling and
finally cold rolled to the final thickness. The separating
agent of the channel regions left out of the welding, which
is applied in the screen printing method, is blown out
using compressed air before the metal sheets are divided
into the individual evaporator plates. This method has the
disadvantage of the metal sheet thickness change during hot
rolling and in the subsequent cold rolling step, since this
results directly in corresponding metal sheet length
changes. Problems result from this which result in a high
reject rate in the following work steps. The evaporators
must be manufactured from pure aluminum (A1 99.5) in order
to allow the introduction of the channels.
The absorber according to the present invention does not,
however, necessarily have to be made of pure aluminum and
nonetheless may be manufactured easily in a large piece
count. In addition, in interest of cost reduction, a method
for mass production of absorbers with a low reject rate is
to be suggested that requires little energy and opens up a-
large design freedom in regard to the design of the pipe
system. These requirements are not fulfilled by the known
roll) bond method, so that it is less suitable for
manufacturing the absorbers of collectors for solar
installation.
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According to the present invention, the metal sheets
forming the absorber are bonded to one another using an
adhesive. In particular, one-component or two-component
adhesives are used, which are resistant to the heat
transfer medium and maintain their adhesive properties at
least in the temperature range between -30 °C and +200 °C.
In a preferred embodiment of the manufacturing method
according to the present invention, the adhesive is not
first applied after the shaping of the pipe system, but
rather already to the starting material of the absorber,
which is particularly strip-shaped. Strips coated in this
way may be wound up like uncoated strips into a coil
without sticking to one another if it is a temperature-
dependent hot melt adhesive. The adhesive effect only sets
in after heating to a specific temperature.
The joining of the metal sheets through an adhesive allows
the use of metal sheets having the final thickness and
final strength, which has advantageous effects on the
dimensional accuracy of the absorber while simultaneously
reducing the reject rate. The energy use is significantly
reduced in gluing in relation to typical bonding
technologies, such as soldering or welding. For planar
application to the join surfaces, the adhesive may be
rolled on using rollers or spread on using a tool similar
to a doctor blade or spatula . Alternatively to the planar
application, the adhesive may also be sprayed on in lines,
the quantity being metered in such way that no excess
adhesive penetrates into the pipe system after the joining
of the metal sheets to be glued.
The shape of the pipe system is introduced through cold
shaping, particularly through deep drawing or embossing,
through which high cross-sectional reproduction precision
and flexible arrangement of the heat transfer pipes in the
metal sheets of the absorber lying one on top of another
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may be achieved on one side, on both sides, or on alternate
sides as required.
The introduction of the pipe system through deep drawing or
embossing allows the use of aluminum alloys when
manufacturing absorbers instead of the pure aluminum
currently used. Suitable aluminum alloys are, for example,
the aluminum wrought alloys cited in the following:
Al Mg 3
A1 Mg Si 1 or
A1 Cu Mg 1.
In an advantageous embodiment of the present invention, at
least the areas of the metal sheets to be glued are
subjected to a surface treatment. If aluminum sheets are
used, an anodized coating is recommended, which is
generated through anodic oxidation of the aluminum sheet.
For sheet steel, a copper or plastic coating may be applied
as a corrosion protection. Additionally or alternatively,
further mechanical and/or thermal surface treatments may be
performed on the areas to be glued. Mechanical surface
treatments (e.g., brushing) remove contamination and
roughen the surface, which may have advantageous effects on
the strength of the adhesive bond for specific adhesives.
The thermal surface treatment degreases the surface.
The joined metal sheets, which are cut to absorber size,
are additionally bonded to one another in a formfitting
way. This additional bonding fixes the metal sheets until
reaching a minimum hardness of the adhesive and unloads the
adhesive bond during operation of the collector at high
temperatures of the heat transfer medium. For this purpose,
formfitting bonds active in the absorber plane are
generated at multiple locations distributed uniformly on
the absorber area using clinching (toxing), which maintain
the fixing of the metal sheets required for the adhesive
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curing and the stabilization of the absorber under all
operating conditions. The absorbers fixed in this way may
leave the press for the joining procedure again immediately
and, if necessary, pass through a curing furnace or cure to
the required adhesive final strength under normal ambient
conditions.
Depending on the adhesive used, it may be necessary for the
metal sheets mechanically fixed in this way to be
additionally pressed and/or heated on one another. For this
purpose the plates are laid on one another with elastic
intermediate layers to form a stack in order to then cure
for the required time under the pressure of a press and/or
the simultaneous effect of temperature.
After completing curing, any necessary post-processing
follows, such as stamping, bending, flanging, and
lacquering.
A production line for manufacturing an absorber according
to the present invention is illustrated in a side view and
a top view in Figures la, lb. Figures 2 shows a schematic
section through a collector having absorbers according to
the present invention:
The exemplary embodiment shows a two-train production line
in which two metal sheets la, lb are processed in parallel.
The strip-shaped metal sheets la, lb, which are each
uncoiled from a coil 2a, 2b, are, after straightening in a
roller straightening machine 3a, 3b, fed to embossing
stations 4a, 4b, which introduce the shape for the pipe
system through embossing in both metal sheets. If the
absorber pipes are only to be embossed on one side, one of
the embossing stations 4a or 4b may be dispensed with; in
this case, a flat metal sheet is joined to an embossed
metal sheet.
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The adhesive application is subsequently performed in both
trains using a roller 5a, 5b positioned above the line
shape in each case. Only after the adhesive is rolled on
are the strip-shaped metal sheets la, lb cut to the size of
the absorber 8 to be manufactured using shears 6a, 6b in
cutting stations 7a, 7b.
Subsequently, the metal sheets 1a, 1b, manufactured in the
two parallel manufacturing trains and cut to the size of
the absorbers, are joined in a compression mold 9 and fixed
in their position to one another using clinching (toxing)
in a formfitting bond 13 active in the metal sheet plane on
at least two locations lla, 11b.
The absorbers thus fixed leave the compression mold 9 again
immediately and reach a curing station 13 in which they
cure under the pressure of a press 14 and the simultaneous
effect of temperature in batches up. to the required
adhesive final strength. Elastic intermediate layers 15 are
located between the curing absorbers 8, which prevent
damage of the absorber pipes embossed on both sides in the
curing station 13. If the capacity of the curing station 13
may not absorb all absorbers 8 which may be manufactured
from the two coils 2a, 2b, multiple curing stations may be
provided to ensure a continuous production flow.
The transport of the metal sheets la, lb between the
cutting stations 7a, 7b, the compression mold 9, and the
curing station 13 is advantageously performed
automatically, for example, using conveyor means and
clocked gripping and lifting devices, which are not shown
in the figures for reasons of clarity.
The schematic construction of the absorber manufactured
using the manufacturing train shown in Figure 1 results
from the sectional illustration of a collector shown in
Figure 2.
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The flat collector, identified as a whole with 16,
comprises a weatherproof housing 17 having a glass cover
18, through which the solar radiation 19 is incident on the
surface 21 of the absorber 22. The preferably dark-colored
surface 21 largely converts the incident solar radiation 19
into heat, which is dissipated via the pipe system 23
integrated into the absorber 22, of which only two absorber
pipes are shown in cross-section. The shape of the absorber
pipes is introduced through cold shaping into the metal
sheets, which are bonded to one another via an adhesive
layer 25. The heat transfer medium, a frostproof water-
glycol mixture, circulates in the absorber pipes.
An insulation layer 24 positioned below the absorber
prevents heat losses via the floor of the housing 17, while
the air layer enclosed by the glass cover 18 in the
absorber 22 acts as a radiation-transparent thermal
insulation on the top of the absorber.
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List of reference numbers
la,b metal sheets
2a,b coil
3a,b roller straightening machine
4a,b embossing station
5a,b roller
6a,b shears
7a,b cutting stations
8 absorber
9 compression mold
-
lla,b locations
12 formfitting bond
13 curing station
14 press
elastic intermediate layer
16 flat collector
17 housing
18 glass cover
19 solar radiation
-
21 surface of collector
22 absorber
23 pipe system
24 insulation layer
adhesive layer
