U.S. patent application number 14/378044 was filed with the patent office on 2015-01-08 for glass-to-metal joint for a solar receiver.
This patent application is currently assigned to ARCHIMEDE SOLAR ENERGY S.R.L.. The applicant listed for this patent is ARCHIMEDE SOLAR ENERGY S.R.L.. Invention is credited to Thomas Chiarappa, Claudio Raggi.
Application Number | 20150007808 14/378044 |
Document ID | / |
Family ID | 46354129 |
Filed Date | 2015-01-08 |
United States Patent
Application |
20150007808 |
Kind Code |
A1 |
Chiarappa; Thomas ; et
al. |
January 8, 2015 |
GLASS-TO-METAL JOINT FOR A SOLAR RECEIVER
Abstract
A glass-to-metal sealing device of a solar receiver has a metal
collar and a glass cylinder to be sealed together. The glass
cylinder is made of a borosilicate glass having a coefficient of
thermal expansion within the range of [3.1, 3.5]10.sup.-6.degree.
C..sup.-1 in a temperature range of [50, 450].degree. C. The metal
collar s made of an austenitic alloy having a coefficient of
thermal expansion in the range of [3.5, 6.0]10.sup.-6.degree.
C..sup.-1 in the temperature range of [50, 450].degree. C. The end
portion of the metal collar is beveled so as to increase its
mechanical flexibility and, further, the end portion of the metal
collar is processed via a thermal treatment in order to establish a
bond between the metal and the glass surfaces.
Inventors: |
Chiarappa; Thomas; (Perugia,
IT) ; Raggi; Claudio; (Terni, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ARCHIMEDE SOLAR ENERGY S.R.L. |
MASSA MARTANA |
|
IT |
|
|
Assignee: |
ARCHIMEDE SOLAR ENERGY
S.R.L.
MASS MARTAMA
IT
|
Family ID: |
46354129 |
Appl. No.: |
14/378044 |
Filed: |
January 22, 2013 |
PCT Filed: |
January 22, 2013 |
PCT NO: |
PCT/EP2013/051141 |
371 Date: |
August 11, 2014 |
Current U.S.
Class: |
126/652 ;
148/287; 403/345 |
Current CPC
Class: |
F24S 2025/6013 20180501;
F24S 10/45 20180501; F24S 80/70 20180501; F24S 20/20 20180501; Y02E
10/40 20130101; Y02E 10/44 20130101; C03C 27/04 20130101; F24S
40/80 20180501; Y10T 403/70 20150115 |
Class at
Publication: |
126/652 ;
148/287; 403/345 |
International
Class: |
F24J 2/05 20060101
F24J002/05; F24J 2/07 20060101 F24J002/07; F24J 2/46 20060101
F24J002/46 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 9, 2012 |
EP |
12425025.9 |
Apr 26, 2012 |
EP |
12425076.2 |
Claims
1-12. (canceled)
13. A glass-to-metal sealing device of a solar receiver, the device
comprising: a metal collar and a glass cylinder to be sealed to one
another; said glass cylinder being formed of a borosilicate glass
having a coefficient of thermal expansion in a range of [3.1, 3.5]
10.sup.-6.degree. C..sup.-1 in a temperature range of [50,
450].degree. C.; said metal collar being formed of an austenitic
alloy having a coefficient of thermal expansion in a range of [3.5,
6.0] 10.sup.-6.degree. C..sup.-1 in the temperature range of [50,
450].degree. C.; said metal collar having a beveled end portion for
increasing a mechanical flexibility thereof; and said end portion
of said metal collar being processed via a thermal treatment in
order to establish a bond between the metal of said metal collar
and glass surfaces of said class cylinder.
14. The glass-to-metal sealing device according to claim 13,
wherein said beveled end portion of said metal collar is defined by
longitudinal sections having a trapezoidal shape with a minor base
at a free end of said metal collar.
15. The glass-to-metal sealing device according to claim 14,
wherein a ratio between a length of the minor base and a length of
a major base of the trapezoidal shape is greater than 0.25.
16. The glass-to-metal sealing device according to claim 15,
wherein the length of the major base lies in a range of [0.3, 0.6]
mm and the length of the minor base lies in a range of [0.15, 0.3]
mm.
17. The glass-to-metal sealing device according to claim 14,
wherein lateral sides (L1, L2) of the trapezoidal shape enclose an
angle in a range of [0.5, 10] degrees.
18. The glass-to-metal sealing device according to claim 13,
wherein said austenitic alloy of said metal collar has a
concentration of nickel and cobalt contents to satisfy an ASTM
F15/DIN 17745 norm.
19. The glass-to-metal sealing device according to claim 13,
wherein the thermal treatment processing said end portion of said
metal collar is an oxidation treatment generating on the metal
surface a glass-dedicated oxide layer.
20. The glass-to-metal sealing device according to claim 19,
wherein the glass-dedicated oxide layer has a thickness spanning a
range of [0.3, 3.0] .mu.m and a penetration in a metal matrix in a
range of [1.5,18.0] .mu.m.
21. The glass-to-metal sealing device according to claim 20,
wherein the glass-dedicated oxide is an iron oxide.
22. The glass-to-metal sealing device according to claim 21,
wherein the iron oxide is selected from the group consisting of
FeO, Fe.sub.3C.sub.4, and a mixture of FeO and Fe.sub.3C.sub.4.
23. A method of producing a glass-to-metal sealing device of a
solar receiver, the device including a metal collar and a glass
cylinder to be sealed together, the method comprising the following
steps: a) providing, as a glass for the glass cylinder, a
borosilicate glass having a coefficient of thermal expansion in a
range of [3.1, 3.5] 10.sup.-6.degree. C..sup.-1 in a temperature
range of [50, 450].degree. C.; b) providing, as a metal of the
metal collar, an austenitic alloy having a coefficient of thermal
expansion in a range of [3.5, 6.0] 10.sup.-6.degree. C..sup.-1 in
the temperature range of [50, 450].degree. C.; c) beveling an end
portion of the metal collar so as to increase a mechanical
flexibility thereof; d) processing the end portion of the metal
collar via a thermal treatment for establishing a bond between the
metal and glass surfaces; and e) sealing together the end collar
portions of the glass cylinder and the metal collar.
24. A tubular solar receiver, comprising an outer glass tube and an
inner metal tube connected to one another via the glass-to-metal
sealing device according to claim 13.
Description
[0001] The present invention relates to a glass-to-metal sealing
device, to a method of producing a glass-to-metal sealing device
and to a tubular solar receiver according to the preambles of
claims 1, 11 and 12 respectively.
[0002] A key component of parabolic trough CSP (Concentrated Solar
Power) is the Heat Collector Element (HCE) also known as solar
receiver. One of the main issues that this element has to face and
solve is the tightness to preserve a suitable designed vacuum
pressure in order to reduce thermal losses to radiative phenomena
only.
[0003] In a solar receiver, the most critical component undergoing
possible vacuum losses is the connection between glass and metal,
also known as Glass-to-Metal Seal (GMS).
[0004] Parabolic trough CSP solar plants are designed to produce
energy by concentrating solar rays to a solar receiver, into which
an Heat Transfer Fluid (HTF) flows through; the transfer fluid
being heated up to high temperatures (up to 580-600.degree. C.) and
allowing, in a separate power block, the production of steam and
therefore of electricity via a dedicated turbine.
[0005] For the thermodynamic cycle to properly work, the solar
receiver has to maximally absorb the concentrated solar rays and
minimally release the heat. A spectral selective coating covering
the stainless steel tube is optimized in order to achieve high
absorbance and low emissivity; furthermore, the thermal loss is
minimized by encapsulating the tube into a vacuum environment by
means of a co-axial cylindrical glass tube (having high optical
transmittance).
[0006] Vacuum is mandatory to reduce thermal losses to radiative
phenomena only.
[0007] A solar receiver will then necessarily contain two
glass-to-metal transitions, also known as Glass to Metal Seals
(GMS), which indeed represent the most critical component to
possible vacuum losses.
[0008] The GMS solutions developed for solar receiving tubes in the
CSP world have been driven both by technology requirements as well
as by market and business needs.
[0009] FIG. 1 is schematically illustrating a solar receiver
comprising an inner metal tube, not shown, and outer glass tube
which are connected together via two glass-to-metal seals 10 and
via two metallic bellows, not shown, welded to the inner metallic
tube. Each glass-to-metal seal 10 comprises a metal collar 11, also
known as metal cap or metal ring, and a glass cylinder 12 sealed
together as schematically shown in FIG. 3. The glass cylinder 12 is
connected to the central glass portion 13 of the outer glass
tube.
[0010] Several different types of glass-to-metal seals with various
glasses and metals with different thermal expansion coefficients
and sealing techniques are known in the art.
[0011] As used herein, the thermal expansion coefficient (TEC) of a
material is defined as the ratio between the elongation, .DELTA.L,
and the proper length, L, of a material when it undergoes a
temperature change .DELTA.T.
[0012] According to the disclosure of two US patents by Mr.
Houskeeper in 1919 (U.S. Pat. No. 1,293,411 and U.S. Pat. No.
1,295,466), it is known a technique for compensating for the
drawbacks caused by the difference in the TEC coefficients of the
glass and the metal, in which the hermetically sealing between
glass and metal is improved by reducing the thickness of a portion
of the metal element with a geometry as proposed by Mr.
Houskeeper.
[0013] According to a known GMS technique, it has been developed a
GMS joint between a stainless steel grade (aisi430) with a
borosilicate glass of the family 3.3. Unfortunately, such GMS joint
between Aisi430 steel and 3.3 borosilicate glass suffers for the
drawback of having a very large difference between the values of
the thermal expansion coefficients (TEC) of the metal and the
glass, with a negative impact on the GMS under mechanical forces
induced by thermal variations. In fact, the TEC values are: almost
constant to 3.310.sup.-6.degree. C.-1 for the glass and between
[10,12]10.sup.-6 .degree. C.-1 for the metal in the temperature
range of [50,450].degree. C.
[0014] According to another known GMS technique (U.S. Pat. No.
7,562,655), it has been developed a GMS joint between an austenitic
alloy with well defined concentrations of Nickel and Cobalt
(commonly known as Kovar-like alloy, DIN 17745, ASTM F15) with a
borosilicate glass of the family 5.1. Unfortunately, such GMS joint
has the drawbacks that Kovar is a pretty costly alloy (oscillating
with Nickel market price fluctuations) and that the 5.1 glass
satisfying CSP dimensional specifications is still uncommon in the
glass market.
[0015] According to other known techniques, transition glasses are
adopted in GMS joints to limit the 5.1 glass to the sole GMS part
(10), hence joining together a kovar-to-5.1 solution to a 3.3 glass
as shown in FIG. 2. FIG. 2 is a drawing schematically illustrating
a portion of the outer glass tube of the solar receiver comprising
a matched GMS joint 10 with a different glass for the central glass
portion 13 by employing a set of transition glasses having
different TECs, as for instance, a 1st transition glass 21, a 2nd
transition glass 22 and a n-th transition glass 23, positioned
between the GMS joint 10 and the central portion 13 of the glass
tube. Unfortunately, such GMS joints have the drawbacks that
transition glasses are expensive and the manufacturing process is
complex.
[0016] Another known sealing technology, as for instance laser
welding, is comfortable but even more sensible to raw materials
tolerances and dimensional specifications.
[0017] It is therefore the aim of the present invention to overcome
the above mentioned drawbacks, in particular by providing a
glass-to-metal sealing device, a method for producing a
glass-to-metal sealing device and a tubular solar receiver
different from a fully matched solution (as for instance the
expensive and market uncommon kovar-to-5.1 glass) and from a
transition glass solution (characterized by cheaper glass used only
for the central glass portion 13) via a direct GMS joint between a
[3.1,3.5] TEC borosilicate glass and an austenitic alloy having
different thermal expansion coefficients.
[0018] The aforementioned aim is achieved by a glass-to-metal
sealing device of a solar receiver, the device comprising a metal
collar and a glass cylinder to be sealed together, the device
further comprising the following features: [0019] a) the glass
cylinder is made out of a borosilicate glass having a thermal
expansion coefficient in the range of [3.1,3.5]10.sup.-6.degree.
C..sup.-1 in the temperature range of [50,450].degree. C.; [0020]
b) the metal collar is made of an austenitic alloy having a thermal
expansion coefficient in the range of [3.5,6.0]10.sup.-6.degree.
C..sup.-1 in the temperature range of [50/450].degree. C.; [0021]
c) the end portion of the metal collar is beveled so as to increase
its mechanical flexibility; [0022] d) the end portion of the metal
collar is processed via a thermal treatment in order to establish a
bond between the metal and the glass surfaces.
[0023] The aforementioned aim is achieved also by a method of
producing a glass-to-metal sealing device of a solar receiver, the
device comprising a metal collar and a glass cylinder (12) to be
sealed together, the method comprising the following steps: [0024]
a) providing, as glass for the glass cylinder, a borosilicate glass
having a thermal expansion coefficient in the range of
[3.1,3.5]10.sup.-6.degree. C..sup.-1 in the temperature range of
[50,450].degree. C.; [0025] b) providing, as metal of the metal
collar, an austenitic alloy having a thermal expansion coefficient
in the range of [3.5,6.0]10.sup.-6.degree. C..sup.-1 in the
temperature range of [50/450].degree. C.; [0026] c) beveling the
end portion of the metal collar so as to increase its mechanical
flexibility. [0027] d) processing the end portion of the metal
collar via a thermal treatment for establishing a bond between the
metal and the glass surfaces; [0028] e) sealing together the end
collar portions of the glass cylinder and the metal collar.
[0029] The aforementioned aim is achieved also by a tubular solar
receiver in which the outer glass tube is connected to the inner
metal tube via the glass-to-metal sealing device according to the
proposed invention.
[0030] Embodiments of the invention enable to retain the designed
vacuum for the expected lifetime of the Heat Collector Element
(HCE). Advantageously, the joint between the glass cylinder and the
metal cap should be able to preserve a designed ultimate desired
tightness, i.e. a pressure of p<10.sup.-4 mbar, by fulfilling
dedicated dimensional requirements so that the stability and
durability of GMS joint is reliably ensured.
[0031] With embodiments of the invention, the glass component of
the GMS joint undergoes mainly compressive stresses, reducing the
dangerous tensile stresses to few MPa which is perfectly acceptable
even by ordinary 3 mm thick glass tubes.
[0032] With embodiments of the invention, the dimensional output
leads to a GMS product perfectly consistent with the typical
working conditions of a solar plant, hence suitable for CSP
applications.
[0033] Embodiments of the invention enable to achieve a
simplification in the manufacturing process, both from the cost
point of view as well as from the final performances achievable for
the target solar receiver product.
[0034] Embodiments of the invention lead to industrial benefits in
the field of unmatched GMS products for the following reasons:
[0035] commodity of raw material: the 3.3 borosilicate glass is a
well know material and easy to find on the market; [0036]
performances of the glass: the 3.3 borosilicate glass easily
reaches a transmittance of 91.5-92.0%; [0037] cost of the raw
material: the 3.3 borosilicate glass is cheaper than the 5.1
borosilicate glass; [0038] simplification of manufacturing process:
the manufacturing process does not contain serious bottleneck steps
so that manufacturing costs are reduced.
[0039] Hence, embodiments of the invention lead to sensible cost
reductions for solar receivers, contributing to the decrease of the
evaluated Levelized Cost Of Energy (LCOE) for solar energy in CSP
parabolic trough plants.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 a drawing schematically illustrating a solar receiver
(Prior Art, previously described);
[0041] FIG. 2 a drawing schematically illustrating a portion of a
solar receiver comprising a GMS device and a set of transition
glasses (Prior Art, previously described);
[0042] FIG. 3 a drawing schematically illustrating a GMS joint
(Prior Art);
[0043] FIG. 4 a drawing schematically illustrating a metal collar
according to an example embodiment of the present invention;
[0044] FIG. 5 a drawing schematically illustrating a GMS device
according to an example embodiment of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0045] FIG. 4 is a drawing schematically illustrating a metal
collar according to an example embodiment of the present
invention.
[0046] FIG. 4(b) is a drawing schematically illustrating the end
portion of the longitudinal section of the metal collar 11 circled
in FIG. 4a.
[0047] According to the proposed present invention, the proposed
GMS joint 10 between the metal collar 11 and the glass cylinder 12
is an unmatched glass-to-metal sealing. In fact, the employed glass
and metal materials behave differently under thermal gradients,
especially when a physical connection between them, i.e. the
sealing, has been established. The closer the corresponding TCE
values, the softer the mechanical stress on the overlap region.
Additionally, the heating up or cooling down velocity is surely
different for glass and metal materials, independently whether
matched or unmatched seals are considered.
[0048] The glass cylinder 12 is made out of a borosilicate glass
having a thermal expansion coefficient in the range of
[3.1,3.5]10.sup.-6 .degree. C..sup.-1 in the temperature range of
[50,450].degree. C. In a preferred embodiment, for the CSP field, a
3.3 borosilicate glass is used. Advantageously, such glass type is
easy to find in the market at commodity prices. The metal collar 11
is made out of an austenitic alloy having a thermal expansion
coefficient in the range of [3.5,6.0]10.sup.-6.degree. C..sup.-1 in
the temperature range of [50,450].degree. C. In a preferred
embodiment of the invention, for the CSP field, such austenitic
alloy has suitable concentration of Nickel and Cobalt contents,
according to the DIN 17745/ASTM F15 norms. In the field, the metals
which fulfill such specifications are also known as Kovar-like
alloys.
[0049] The end portion of the metal collar 11 is beveled so as to
increase its mechanical flexibility. Advantageously, with such
developed metal collar geometry, the tensions originated on the
glass side of the GMS joint 10 are decreased through a compensation
of the glass stiffness with respect to the metal mobility.
[0050] Thus, such metal collar geometry characterized by elastic
properties mitigates the glass stresses which can be produced on
the (stiff) glass component of the GMS joint by improving the
mechanical elasticity of the metal.
[0051] Drawings of preferred embodiment examples are schematically
illustrated in FIG. 4 and in FIG. 5.
[0052] The illustrated dimensions of FIG. 4 and FIG. 5 have been
obtained by studying the mechanical stresses involved in real
working conditions of a typical solar CSP plant. Preferred used
materials are a 3.3 borosilicate glass for the cylinder 12 and a
Kovar-like alloy (DIN 17745 , ASTM F15) for the metal collar
11.
[0053] FIG. 4(a) schematically illustrates a metal collar 11
according to an example embodiment of the present invention.
[0054] FIG. 4(b) schematically illustrates a zoomed detail of the
longitudinal section of the free end portion FEP of the metal
collar 11 as circled in FIG. 4(a). Such metal collar free end
portion FEP will be then sealed to the glass cylinder as later
described.
[0055] According to a preferred embodiment, as schematically shown
in FIG. 4(b), the beveling of the end portion of the metal collar
11 is performed so as to obtain longitudinal sections having a
trapezoidal-like shape in which the minor base m is at the free end
side of the metal collar. It is noted that, herein, with the term
trapezoidal-like shape it is not intended only the trapezium shape
itself but also similar shapes in which the sides are not totally
straight or in which the two bases are parallel but more a bevel
shape or a tooth-like shape in which the end portion has a reduced
thickness with respect to the beginning portion. In fact, the term
trapezoidal-shape has been herein used mainly for explanatory
purposes, i.e. for the sake of simplicity so as to describe the
geometrical specifications in terms of bases and angles. In FIG.
4(c) are illustrated the major base M, the minor base m, the two
lateral sides L1, L2 of the trapezoidal-like shape of the collar
end. The lateral side L1 is also representing the distance between
the two bases m,M. The angle .alpha., not shown, indicates the
acute angle formed by the two lateral side L1, L2.
[0056] In invention embodiments, the following dimensions are
recommended based on studies on real stress conditions of typical
CSP plants: [0057] the ratio between the length of the minor base m
and the length of the major base M being greater than 0.25; and/or,
[0058] the length of the major base M in the range of [0.3, 0.6] mm
and the length of the minor base m being in the range of [0.15,
0.6] mm; and/or, [0059] the lateral sides L1,L2 forming an angle
.alpha. in the range of [0.5,10] degrees; and/or, [0060] the
maximum thickness T of the metal collar being in the range of
[0.3,1.2] mm.
[0061] For example, in a preferred embodiment, the length of the
minor base m may be 0.3, the length of the major base M may be 0.4,
the length of the lateral side L1 may be 7 mm and the angle .alpha.
may be 0.82 degrees.
[0062] According to the proposed invention, the end portion of the
metal collar 11 is processed via a thermal treatment for
establishing a bond between the metal and the glass surface of the
GMS joint. Advantageously, a dedicated structure on the metal
surface suitable for a physical and chemical bond of the metal to
the glass is achieved.
[0063] In fact, with such thermal treatment of the metal collar, a
grid structure on the metal is conveniently created so that the
glass material grips to the metal substrate (mechanical/physical
join) and, simultaneously, a proper layer is suitably created on
the metal surface in order for the glass to bond to it (chemical
bond).
[0064] According to a preferred embodiment of the invention, the
thermal treatment may preferably be an oxidation treatment to
generate on the metal surface a glass-dedicated oxide layer.
[0065] Preferably, the glass-dedicated oxide layer thickness is
tuned to be in the range of [0.3,3.0] .mu.m, with a penetration in
the metal matrix in the range of [1.5,18.0] .mu.m.
[0066] Additionally, according to another preferred embodiment, it
is recommended to develop an oxidation process characterized by a
hydrogen content limited to few percents in concentration (upto 5%
in volume), in order to minimize the sticking of hydrogen-atoms in
interstitial position within the crystalline structure (as hydrogen
is one of the most difficult gas to be pumped away).
[0067] According to a preferred embodiment of the invention, the
glass-dedicated oxide is an iron oxide. The iron oxide may
preferably be either FeO or Fe.sub.3O.sub.4 or a mixture of FeO and
Fe.sub.3O.sub.4.
[0068] A controlled thermal cycle process is advised in order to
achieve the desired iron oxide as well as the optimum thickness and
uniformity.
[0069] According to a preferred embodiment, the following thermal
sealing process steps may be recommended for sealing the metal
collar to the glass cylinder: [0070] a heating step, in which
controls on temperature and on rotation are recommended; [0071] a
melting step, in which controls on temperature, on rotation and on
calibration of the molten glass edge are recommended; [0072] a
joining step for inserting the beveled metal into the molten end
portion of the glass cylinder, in which controls on temperature, on
rotation, on burner relative positions and on mechanical produced
forces (push/pull) on the will be final GMS joint are recommended;
[0073] an in-line annealing step, in which careful control on
temperature decrease to achieve a glass temperature below its
characteristic softening temperature is recommended.
[0074] In order to avoid destructive effects due to small error
propagations, the most critical thermal sealing sub-steps have been
monitored and accordingly some parameters have been identified as
requiring particular attention in controlling their absolute values
and behaviors as, for example: [0075] heating up rates should
preferably be maintained within [6,35].degree. C./sec, [0076]
cooling down rates should preferably be maintained within
[1.5,20].degree. C./sec, [0077] rotational speed should preferably
be maintained within [12,100] rpm, [0078] burner should preferably
be adjustable in a distance range of [-5.5,5.5]mm from the glass
edge, depending on the considered process step, and at a
translational speed within [0,15] mm/sec.
[0079] FIG. 5(b) is a drawing schematically illustrating, according
to an example embodiment, the circled detail of the GMS joint of
FIG. 5(a).
[0080] According to a preferred embodiment, the end portion of the
glass cylinder 12 is melted via a dedicated thermal process so as
to form, at the glass edge, an enlarged molten glass having a
sphere-like shape, hereinafter denoted as molten glass ball GB. As
shown in FIG. 5(b) the glass edge has a maximum thickness of circa
12.6 mm (Gi+Ge+m) while the thickness G.sub.T of the glass cylinder
away from the glass ball GB is a regular glass thickness of circa 3
mm.
[0081] In preferred embodiments, the following dimensions for the
GMS joints are advantageously recommended, where we denote by
"internal" side and by "external" side the side looking towards the
symmetry axis of the solar tubular receiver and the side looking
towards the outer atmosphere, respectively: [0082] Axial (linear)
extensions of the glass-to-metal overlap internal and external
O.sub.i, O.sub.e: in the range of [2,5.0] mm, and/or [0083] Glass
thicknesses internal and external G.sub.i, G.sub.e in the range of
[3.0,6.0] mm; and/or, [0084] Moreover, it is also recommended to
end up with a smooth, smaller than 90.degree. contact angle
.beta..sub.i,.beta..sub.edefined as the angles measured at the
contact interfaces between glass and metal in FIG. 5(b).
[0085] Since the previously described thermal sealing process
involves very high temperatures on the metal collar and on the
glass cylinder (exceeding the softening and the melting points),
unavoidable tensions might get stacked at the interface between the
two materials.
[0086] As these tensions could bring to GMS micro breakage, i.e.
leaking of the GMS, during the further thermal cycles typical of
successive solar receiver production steps as well as in real life
working conditions, it is recommended, in a preferred embodiment,
to implement an off-line annealing process whose goal is to get rid
of possible glass residual stresses in the overlap region.
[0087] Hence, the above mentioned tensions can be advantageously
smeared in their intensity over a wider region, decreasing
therefore their potential dangerous impact. The cooling down rate
value may conveniently be set in the range of [0.4,2.5].degree.
C./min.
[0088] In embodiments of the invention, as part of a vacuum device,
the glass cylinders 12 and the metal caps 11 may be properly
cleaned, by developing a dedicated recipe in order to avoid
undesired contaminants, however without applying strong chemical
polishing which can cause unwanted nano-scratches on the glass
surfaces. Under cleaning procedure should also be intended the ways
the GMS joints are stored, aiming at avoiding contamination with
humidity and greasy atmosphere during the storing procedure.
[0089] In addition to the embodiments of the present invention
described above, the skilled persons in the art will be able to
arrive at a variety of other arrangements and steps which, if not
explicitly described in this document, nevertheless fall within the
scope of the appended claims.
LIST OF USED ACRONYMS
[0090] CSP Concentrated Solar Power
[0091] GMS Glass-to-Metal Seal
[0092] HCE Heat Collector Element
[0093] HTF Heat Transfer Fluid
[0094] LCOE Levelized Cost Of Energy
[0095] TEC Thermal Expansion Coefficient
* * * * *