U.S. patent application number 11/562164 was filed with the patent office on 2007-10-11 for tubular radiation absorbing device for a solar power plant with reduced heat losses.
Invention is credited to Nikolaus Benz, THOMAS KUCKELKORN.
Application Number | 20070235023 11/562164 |
Document ID | / |
Family ID | 38047552 |
Filed Date | 2007-10-11 |
United States Patent
Application |
20070235023 |
Kind Code |
A1 |
KUCKELKORN; THOMAS ; et
al. |
October 11, 2007 |
TUBULAR RADIATION ABSORBING DEVICE FOR A SOLAR POWER PLANT WITH
REDUCED HEAT LOSSES
Abstract
The tubular radiation absorbing device (1) for solar thermal
applications includes a central tube (3) made of chromium steel,
particularly stainless steel; a glass tubular jacket (2)
surrounding the central tube so as to form a ring-shaped space (6);
and a barrier coating (4) on at least an interior side of the
central tube (3), which is substantially impermeable to hydrogen
and contains chromium oxide. The barrier coating (4) is provided by
a process in which the central tube (3) is treated with steam
containing free hydrogen at a temperature of 500.degree. C. to
700.degree. C.
Inventors: |
KUCKELKORN; THOMAS; (Weiden,
DE) ; Benz; Nikolaus; (Weiden, DE) |
Correspondence
Address: |
MICHAEL J. STRIKER
103 EAST NECK ROAD
HUNTINGTON
NY
11743
US
|
Family ID: |
38047552 |
Appl. No.: |
11/562164 |
Filed: |
November 21, 2006 |
Current U.S.
Class: |
126/652 |
Current CPC
Class: |
F24S 40/40 20180501;
F24S 40/46 20180501; F24S 10/45 20180501; Y02E 10/40 20130101; Y02E
10/44 20130101; F24S 20/20 20180501 |
Class at
Publication: |
126/652 |
International
Class: |
F24J 2/50 20060101
F24J002/50 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 25, 2005 |
DE |
10 2005 057 277.4 |
Claims
1. A tubular radiation absorbing device (1) for solar thermal
applications, especially for a parabolic trough collector in a
solar power plant, said radiation absorbing device comprising a
central tube (3) comprising steel, said steel including chromium; a
tubular jacket (2) comprising glass and surrounding the central
tube so as to form a ring-shaped space (6) between the tubular
jacket and the central tube; and a barrier coating (4) on at least
an interior side of the central tube (3), wherein said barrier
coating (4) is substantially impermeable to hydrogen and contains
chromium oxide.
2. The tubular radiation absorbing device as defined in claim 1,
wherein said steel comprises a stainless steel.
3. The tubular radiation absorbing device as defined in claim 1,
wherein said barrier coating (4) has a thickness of 0.5 .mu.m to 10
.mu.m.
4. The tubular radiation absorbing device as defined in claim 1,
wherein said barrier coating (4) contains from 20 wt. % to 60 wt. %
of said chromium oxide.
5. The tubular radiation absorbing device as defined in claim 1,
further comprising an outer coating (5) on an outer side of said
central tube (3), and wherein said outer coating comprises said
chromium oxide.
6. The tubular radiation absorbing device as defined in claim 4,
wherein said outer coating (5) has a thickness that is smaller than
a thickness of said barrier layer.
7. The tubular radiation absorbing device as defined in claim 5,
wherein said thickness of said outer coating (5) is less than or
equal to 0.1 .mu.m.
8. A process for making a central tube (3) of a tubular radiation
absorbing device (1) for solar thermal applications, said process
comprising the steps of: a) prefabricating a central tube (3) made
of chromium-containing steel; and b) treating at least an interior
side of the central tube (3) with free-hydrogen-containing steam at
a temperature of from 500.degree. C. to 700.degree. C. in order to
provide at least said interior side of said central tube with a
barrier coating (4), wherein said free-hydrogen-containing steam
comprises water and free hydrogen and said barrier coating (4)
contains chromium oxide and is substantially impermeable to said
free hydrogen.
9. The process as defined in claim 8, wherein said
chromium-containing steel comprises stainless steel.
10. The process as defined in claim 8, wherein an outer side of the
central tube (3) is treated with another free-hydrogen-containing
steam containing said water and said free hydrogen in a ratio
(V.sub.A) of said free hydrogen to said water that is greater than
a ratio (V.sub.1) of said free hydrogen to said water in said
free-hydrogen-containing steam that treats said interior side of
said central tube (3).
11. The process as defined in claim 10, wherein said ratio
(V.sub.A) of said free hydrogen to said water in said another
free-hydrogen-containing steam is from 10 to 1000, said ratio
(V.sub.1) of said free hydrogen to said water in said
free-hydrogen-containing steam is from 1 to 100, and said ratio
(V.sub.A) of said free hydrogen to said water in said another
free-hydrogen-containing steam is greater than or equal to ten
times said ratio (V.sub.1) of said free hydrogen to said water in
said free-hydrogen-containing steam.
12. The process as defined in claim 8, further comprising working
said central tube (3) on an outer side of the central tube so that
surfaces of said outer side have a surface roughness (R.sub.A) less
than 0.3 prior to treating with said steam.
13. The process as defined in claim 12, wherein said working
comprises polishing.
14. The process as defined in claim 8, further comprising working
said central tube (3) on an outer side of the central tube so that
surfaces of said outer side have a surface roughness (R.sub.A) less
than 0.25 prior to treating with said steam.
15. The process as defined in claim 14, wherein said working
comprises polishing.
Description
CROSS-REFERENCE
[0001] The invention described and claimed hereinbelow is also
described in German Patent Application 10 2005 057 277.4-15, filed
Nov. 25, 2005 in Germany, which provides the basis for a claim of
priority under 35 U.S.C. 119.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a tubular radiation
absorbing device for solar thermal applications, especially for a
parabolic trough collector in a solar power plant, which comprises
a central tube made from a chromium steel, especially stainless
steel, and a glass tubular jacket surrounding the central tube so
as to form a ring-shaped space between the tubular jacket and the
central tube.
[0004] 2. Related Art
[0005] Tubular radiation absorbing devices or absorber pipes are
used in parabolic trough collectors to utilize solar radiation. The
solar radiation is concentrated by a tracking mirror on a tubular
radiation absorbing device and converted into heat. The heat is
conducted away by a heat-carrying medium passing through the
tubular radiation absorbing device and is used directly as process
heat or converted into electrical energy.
[0006] This sort of tubular radiation absorbing device typically
comprises a coated central tube and a glass tubular jacket around
it. The ring-shaped space between the tubes is evacuated. In
operation a heat carrier fluid, especially an oil, is pumped
through the central tube.
[0007] This sort of absorber tube is described, e.g., in DE 102 31
467 B4. A glass-metal transitional element is arranged at the free
end of a glass tubular jacket. The central tube and the glass-metal
transitional element are connected with each other so that they are
slidable relative to each other in a longitudinal direction by
means of at least one expansion compensating device.
[0008] Free hydrogen, which is dissolved in the heat carrier
medium, is generated during aging of the heat carrier fluid. This
hydrogen arrives in the evacuated ring-shaped space between the
central tube and the glass tubular jacket by permeation through the
central tube. The permeation rate increases with increasing
operating temperature, which is between 300.degree. C. and
400.degree. C., so that the pressure in the ring-shaped space
rises. This pressure increase leads to increased heat losses and to
a reduced efficiency of the tubular radiation absorbing device.
[0009] Suitable measures must then be taken to maintain a vacuum in
the ring-shaped space. One measure that is taken to remove hydrogen
is to combine it with a suitable material.
[0010] Getter material, which combines with the hydrogen gas that
penetrates through the central tube into the ring-shaped space, is
arranged in the ring-shaped space to maintain the vacuum. When the
capacity of the getter material is exhausted, the pressure rises in
the ring-shaped space until the partial pressure of the free
hydrogen in the ring-shaped space reaches equilibrium with the
hydrogen dissolved in the heat carrier medium. The equilibration
pressure of the hydrogen in the ring-shaped space amounts to
between 0.3 mbar and 3 mbar in the known absorber tubes. There is
an increase in heat conduction in the ring-shaped space because of
the presence of hydrogen in it. The heat losses due to heat
conduction are about five times higher compared to air, i.e.
clearly higher than with an absorber tube that has not been
evacuated.
[0011] A getter arrangement is described in WO 2004/063640 A1, in
which a getter strip is arranged between the central tube and the
tubular jacket in the ring-shaped space. This arrangement has the
disadvantage that the strip is in a region, which can be exposed to
direct radiation. The getter strip can be heated especially by
radiation coming from the mirror that misses the central tube or
strikes it but is largely reflected from it. Since the getter strip
is nearly thermally isolated from the central tube and the tubular
jacket in a vacuum, the temperature of the getter strip can vary
greatly with the varying radiating conditions. Because the getter
material with a predetermined loading degree has a temperature
dependent equilibrium pressure (equilibrium between gas desorption
and adsorption), temperature fluctuations of the getter material
lead to undesirable pressure fluctuations. The temperature of the
tubular jacket greatly increases after consumption of the getter
material and the absorber tube becomes unusable.
[0012] A chromium oxide coating or layer has been provided on
chromium-containing steel according to "Initial oxidation and
chromium diffusion. I. Effects of surface working on 9-20%-Cr
Steels" by Ostwald and Grabke, Corrosion Science 46, pp. 1113-1127
(2004) in order to protect the steel from a reactive environment.
The chromium-containing steel is provided with a coating by means
of an H.sub.2--H.sub.2O atmosphere, which comprises an inner layer
of Cr.sub.2O.sub.3 and an outer layer of (Mn,Fe)Cr.sub.2O.sub.4
spinel.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide a
tubular radiation absorbing device that has lower heat losses than
conventional tubular radiation absorbing devices of the prior
art.
[0014] This object is attained by a tubular radiation absorbing
device, in which the central tube has a barrier coating that is
largely impermeable to hydrogen, at least one its interior side.
This barrier coating contains chromium oxide, Cr.sub.2O.sub.3.
[0015] It has been surprisingly found that coatings containing
chromium oxide largely prevent passage of hydrogen.
[0016] The hydrogen diffusion from the interior of the central tube
to the ring-spaced space could be reduced by a factor of up to 50
by this barrier coating.
[0017] The coating having chromium oxide is obtained by treating
the central tube comprising steel, especially stainless steel, in a
process in which a surface layer of the central tube is converted
into a coating containing chromium oxide.
[0018] The barrier coating has a preferred coating thickness of 0.5
.mu.m to 10 .mu.m. The barrier action of the barrier coating
decreases when the coating thickness is smaller than the foregoing
preferred coating thickness. Crack formation increases in coatings
that are thicker than this preferred coating thickness due to
temperature changes, so that the barrier action similarly decreases
when the coating thickness is thicker than the foregoing preferred
coating thickness.
[0019] The chromium oxide content of the barrier coating is
preferably from 20 wt. % to 60 wt. %, especially 30 wt. % to 50 wt.
%. The chromium oxide fraction is determined by the chromium
content of the steel and the type and duration of the treatment of
the central tube, as explained in connection with the claimed
process. The barrier action for hydrogen is initiated at a chromium
oxide content of 20 wt. %.
[0020] Preferably the central tube has an outer coating on its
outside which contains chromium oxide.
[0021] However it is preferred that the thickness of the outer
coating is less than the thickness of the barrier coating. This
coating merely serves as an adherent layer for a subsequently
applied selective thin layer. The thickness of the outer layer
amounts to preferably less than 0.1 .mu.m. It has been shown that a
spinel layer, which has a rough surface and is porous, is formed on
the upper surface of the chromium oxide coating with a layer
thickness of greater than 0.1 .mu.m. This spinel layer is not
suitable to support a subsequently applied smooth selective thin
layer. The spinel layer does not interfere with the interior
barrier coating, so that greater thickness is possible.
[0022] The process for making a central tube from steel containing
chromium, especially from chromium-nickel steel, comprises first
prefabricating a central tube from the steel, especially stainless
steel, and then subjecting this central tube to a steam oxidation,
in which the central tube is treated with steam containing free
hydrogen at temperatures of from 500.degree. C. to 700.degree. C.
in order to provide a barrier coating that is largely impermeable
to hydrogen, at least on the interior side of the central tube.
[0023] Preferably the ratio V.sub.A=H.sub.2/H.sub.2O of the steam
for treating the outer side of the central tube is greater than the
ratio V.sub.1=H.sub.2/H.sub.2O of the steam for treating the inner
side of the central tube. The formation of the spinel layer on the
outer side is avoided by these process steps.
[0024] A preferred ratio V.sub.A is from 10 to 1000, while a
preferred ratio V.sub.1 is from 1 to 100. However in this case
V.sub.A.gtoreq.V.sub.1.
[0025] According to another embodiment the coating thickness on the
outer side can be reduced so that the central tube is worked or
process on its outside prior to the steam treatment, so that it has
a roughness R.sub.a less than 0.3. Preferably the roughness R.sub.a
is less than 0.25.
[0026] A polishing procedure can be performed on the outer side of
the central tube in order to perform this treatment.
[0027] In this second embodiment however the use of different
values for the ratios V.sub.A and V.sub.1 is not required, but of
course could be considered as an aid.
BRIEF DESCRIPTION OF THE DRAWING
[0028] The objects, features and advantages of the invention will
now be illustrated in more detail with the aid of the following
description of the preferred embodiments, with reference to the
accompanying FIGURES in which:
[0029] The sole FIGURE is a cutaway longitudinal cross-sectional
view through a preferred embodiment of the tubular radiation
absorbing device according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0030] A tubular radiation absorbing device 1 for solar thermal
applications is shown in the cross-sectional view in the sole
FIGURE. The tubular radiation absorbing device 1 comprises a
central tube 3 made of metal and, a glass tubular jacket 2
surrounding the central tube so that a ring-shaped space 6 is
formed between the tubular jacket and the central tube.
[0031] A heat carrier medium, which contains free hydrogen, flows
through the central tube 3, which is made of metal. The hydrogen
can permeate metal and thus pass through the central tube 3 into
the ring-shaped space 6. In order to prevent the free hydrogen from
passing through the central tube 3, which e.g. is made from
Chromium-Nickel-Molybdenum 17-12-2 Steel No. 1.4404, it is provided
with a barrier coating 4 on its interior side, which contains
Cr.sub.2O.sub.3.
[0032] The inner coating 4 has a thickness of e.g. 10 .mu.m. The
coating 4 comprises a first layer and a further or second layer
applied to the first layer. The first layer contains 30%
Cr.sub.2O.sub.3, from 15 to 18% NiO, and from 50 to 54%
Fe.sub.2O.sub.3. The further or second layer is predominantly
composed of Fe.sub.2O.sub.3, i.e. 98% Fe.sub.2O.sub.3. The chromium
oxide content of the second layer is only about 1 to 2%. This
second layer, which forms the spinel layer, still contains a small
amount of nickel oxide.
[0033] The central tube 3 has an outer coating 5 on its outer side,
which has a thickness of 0.05 .mu.m. This coating 5 has no spinel
layer.
[0034] The preparation of the oxide coatings 4, 5 takes place by
means of a steam oxidation process according to the following
parameters:
[0035] H.sub.2/H.sub.2O ratio for both coatings 4,5,
[0036] Outer surface of the central tube: polished, Ra<0.2
.mu.m,
[0037] Temperature T=500.degree. C., and
[0038] Treatment time: 5 hours.
PARTS LIST
[0039] 1 tubular radiation absorbing device [0040] 2 tubular jacket
[0041] 3 central tube [0042] 4 barrier coating [0043] 5 outer
coating [0044] 6 ring-shaped space
[0045] While the invention has been illustrated and described as
embodied in a tubular radiation absorbing device for a solar power
plant with reduced heat losses, it is not intended to be limited to
the details shown, since various modifications and changes may be
made without departing in any way from the spirit of the present
invention.
[0046] Without further analysis, the foregoing will so fully reveal
the gist of the present invention that others can, by applying
current knowledge, readily adapt it for various applications
without omitting features that, from the standpoint of prior art,
fairly constitute essential characteristics of the generic or
specific aspects of this invention.
[0047] What is claimed is new and is set forth in the following
appended claims.
* * * * *