CA1254465A - Solar selective surface coating - Google Patents
Solar selective surface coatingInfo
- Publication number
- CA1254465A CA1254465A CA000438343A CA438343A CA1254465A CA 1254465 A CA1254465 A CA 1254465A CA 000438343 A CA000438343 A CA 000438343A CA 438343 A CA438343 A CA 438343A CA 1254465 A CA1254465 A CA 1254465A
- Authority
- CA
- Canada
- Prior art keywords
- layer
- infra
- collector element
- solar
- deposited
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
- F24S10/00—Solar heat collectors using working fluids
- F24S10/40—Solar heat collectors using working fluids in absorbing elements surrounded by transparent enclosures, e.g. evacuated solar collectors
- F24S10/45—Solar heat collectors using working fluids in absorbing elements surrounded by transparent enclosures, e.g. evacuated solar collectors the enclosure being cylindrical
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
- F24S70/00—Details of absorbing elements
- F24S70/20—Details of absorbing elements characterised by absorbing coatings; characterised by surface treatment for increasing absorption
- F24S70/225—Details of absorbing elements characterised by absorbing coatings; characterised by surface treatment for increasing absorption for spectrally selective absorption
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
- F24S70/00—Details of absorbing elements
- F24S70/20—Details of absorbing elements characterised by absorbing coatings; characterised by surface treatment for increasing absorption
- F24S70/25—Coatings made of metallic material
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
- F24S70/00—Details of absorbing elements
- F24S70/30—Auxiliary coatings, e.g. anti-reflective coatings
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/40—Solar thermal energy, e.g. solar towers
- Y02E10/44—Heat exchange systems
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S126/00—Stoves and furnaces
- Y10S126/907—Absorber coating
- Y10S126/908—Particular chemical
Landscapes
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Sustainable Energy (AREA)
- Thermal Sciences (AREA)
- Chemical & Material Sciences (AREA)
- Combustion & Propulsion (AREA)
- Life Sciences & Earth Sciences (AREA)
- Sustainable Development (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Surface Treatment Of Glass (AREA)
- Optical Elements Other Than Lenses (AREA)
- Laminated Bodies (AREA)
- Application Of Or Painting With Fluid Materials (AREA)
- Road Signs Or Road Markings (AREA)
Abstract
SOLAR SELECTIVE SURFACE COATING
ABSTRACT OF THE DISCLOSURE
A solar collector element having an inner tube through which a fluid can be passed, an outer glass tube enveloping the periphery of the inner tube and defining an evacuated space between the two tubes, and a solar selective surface coating deposited on the outer surface of the inner tube. The surface coating is deposited as three layers, an inner layer composed of a metal which exhibits high reflectivity in the infra-red spectral range, an outer layer composed of a material which behaves as a semiconductor at collector operating temperatures and an intermediate layer which is composed of a dielectric material. The materials of the respective layers are selected such that the refractive index mismatch between the inner layer and the intermediate layer is greater than it would be between the inner layer and the outer layer, in the absence of the intermediate layer, whereby the infra-red emittance of the collector element is reduced relative to that of an equivalent collector element having a two-layer selective surface coating.
ABSTRACT OF THE DISCLOSURE
A solar collector element having an inner tube through which a fluid can be passed, an outer glass tube enveloping the periphery of the inner tube and defining an evacuated space between the two tubes, and a solar selective surface coating deposited on the outer surface of the inner tube. The surface coating is deposited as three layers, an inner layer composed of a metal which exhibits high reflectivity in the infra-red spectral range, an outer layer composed of a material which behaves as a semiconductor at collector operating temperatures and an intermediate layer which is composed of a dielectric material. The materials of the respective layers are selected such that the refractive index mismatch between the inner layer and the intermediate layer is greater than it would be between the inner layer and the outer layer, in the absence of the intermediate layer, whereby the infra-red emittance of the collector element is reduced relative to that of an equivalent collector element having a two-layer selective surface coating.
Description
~5'~
FIELD OF THE INVENTION
This invention relates ~o a solar selective surface coating for absorber surfaces which are employed in solar collector~. In ~articular. the invention is directed to a solar selective surface coating which includes a refractive index mismatch layer for the purpose of reducing in~ra-red emittance.
BACKGRO~ND OF THE INVENTION
Solar æelective surface coatings known in the art usually comprise an ou~er layer of a material which absorbs strongly in the solar energy spectral range but which is transparent to infra-red radiation. In the interest of reducing ther~al losses, the outer layer is deposited on an inner layer of a material which provide6 hiyh reflectivity and, hence. low emissivity in the infra-red spectrum. A
typical such selective surface coating is disclosed in U.S.
Patent No.4.339,484, the surface coating comprising a metal-carbide solar energy absorp~ive outer layer and a copper infra red reflective base coating. The metal-carbide absorpti~e layer is graded such that it has a high carbide-to-metal ratio near its outer surface and a high metal-to carbide ratio adjacent the interface between the absorptive surface and the reflec~ive base coating.
A problem that is inherent in the traditional selective surface coatings is that the emissivi~y o~ the total sur~ace coating is much higher than the emissivity of the reflective base layer alone. One of the major reasons for this is that 4~i5 the emissivity o~ a material increases if the optical constan~s of a superimposed medium are different from those of free space, and this normally is the case with materials tha~ are employed for absorptive surface coatings. Such ma~erials, if semiconductors. may have refractive indices in the order o~ 2 to lO times greater than that of free space and, there~ore, the coupling of infra-r~d radiation across the metal-semiconductor interface may be significantly greater than tha~ of a metal-air interface.
SUMMARY OF THE INVENTION
The present invention seeks to alleviate this problem by providing a solar selective surface coating which comprises at least three layers: an inner layer composed of a material having high reflectivity and, hence, low emissivity in the infra-red spectral range, an outer layer composed o~ a material which i~ absorptive of energy in the solar energy spectral range and which is substantially ~ransparent to infra-red radiation, and an intermediate layer composed oP a material which i& substantially transparent to in~ra-red radiation. The inner, outer and intermediate layer materials ha~e complex refractive indices (nl-ikl), (n2-ik2~ and (n3-ik3) respectively and the materials are selected to satisfy the relationship r(n3-ik3)-(nl-ikl)l2 r~n2-ik2)-(nl-ikl)l2 L(n3_ik~)+(n~ tn2-ik2)~(nl-ikl~
for normal incident radiation over at least a major portion 6~
of the infra-red spectral range.
With ~he above defined surface coating structure, the re~ractive index mismatch between the inn~er layer and the intermediate layer is greater than it would be between the inner layer and the outer layer (in the absence of the intermediate layer) and, therefore, less infra-red radiation will be coupled ou~ of the inner layer. Stated in an alternative way, a higher value o infra-red reflectance i~
obtainea at the interface of the inner and intermediate layers than would be attainable at the interface of a two-layer solar selective surface coating.
PREFERRED FEATURES OP THE IMVENTION
The intermediate layer preferably is composea of a dielectric ma~erial which is highly transparent to infra-red radiation (i.e., having k3 approximately equal to 0) and which has a real refractive index n3 less than about 2.5.
Suitable such materials include magnesium fluoride, magnesium oxide, titanium oxide, aluminium oxide, silica, quartz and carbon.
The outer layer preferably i6 composed of a m~erial which beha~es as a semiconductor at collector operating temperatures. Such material may ha~e a monocrystalline, polycrystalline or amorphous structure and may comprise, ~or example, germanium, a germanium-sillcon alloy, silicon carbide, lead sulphide, boron or, in the case of surface coatings to be employed in relatively low ~emperatu~e collector systems, tellurium compounds. The material may ~25~ 5 alternatively be in the form of a cermet and, in such case, the mateLial forming the dielectric matrix of the cermet may be the same as or different from the dielectric material which forms the intermediate layer of the surface coating.
Also, the outer layer preferably is graded, either geome~rically or in ~erms of its composi~ion, such that its refractive index to solar radiation increases with increasing depth of ~he layer and such that the maximum ~real and imaginery) refractive indices (n2 and k2~
occur adjacent the interface with the intermediate layer.
Geometrical grading (i.e. texturing) may be achie~ed by chemically etching the outer layer o~ the surface coating.
Furthermore, the outer layer may be constituted by a number of sub-layers and include interference layers for the ~urpose of providing destructive interference to solar radiation. Simila~ly, the intermedia~e layer may include a nu~ber of sub-layers, but preliminary investigations tend to indica~e that no benefit is to be gained from s~ch a structure. ~owever, the point to be noted is that, allowing for the possibility of sub-layers, the total surface structure may, in ~act, include more than three layers.
The inner layer preferably c~mprises a metal, such as copper, aluminium, molybdenum, silver or gold which exhibits a high re~lec~ance to infra-red radiation, and such metal layer, when composed of copper, would normally be deposited to a thickness of at least O.lO x l~ 6m.
The intermediate layer preferably is deposited to a ~2Si4~
minimum thickness of 0.15 x 10 6m., and the outer layer would normally be deposited to produce a solar energy absorptiYe layer having a ~hickness in the order of 0.3 x 10~6m. to 5.0 x 10~6m.
The inner, intermediate and outer layer materials which are employed in any given coating de~irably should have coefficients of thermal expan6ion which are approximately equal, in order that the risk of differecltial ~ovement at the interface betwee~ ~he layers might be minimi6ed.
APPLICATION OF T~E INV~NTION
The solar selective surface coating may be applied to a flat plate-type collector surface. However, the surface coating would normally be applied to a tubular-type collector element having an inner (single or double ended) tube through which a heat exchange medium is caused to flow, an outer glass tube which envelopes the inner tube and an evacuated space between the two tubes. In the case of a tubular-type collector element, the surface coating would be deposited on the outer surface of the inner tube.
The inner tube may be formed from glass or metal.
depending upon the intended operating temperature of the collector system. Glass would normally be employed for operating temperature~ up to about 300C and metal for tempera~ures exceeding 300 C.
When a metal tube is employed, the tube itself may constitute the inner layer of the surface coating and, in such case, the intermediate layer would be deposited directly onto the outer surface of the metal tube. However, ~25~i5 .
a stainless steel or titanium tube would normally be used and, due to the relatively high infra-red emittance of such metals~ an inner layer of a low emi~tance metal would normally be coated onto the tube to form the inner layer.
Various techniques may be employed for aepssiting the respective layers of the surface coating onto the collector tube. For exam~le, electron beam, magnetron spu~tering, r.f. sputtering or, when appropriate, reac~ive sputtering deposition ~echniques may be employed for all layers.
Alternatively, the inner layer may be applied by electroplating, dipping or vapour deposition techniques, whilst the intermediate layer may be applied by dipping and the outer layer may be applied by chemical vapour deposition, The technique employed will depend on the ma~erial used in the respective layers and the material from which the collector tube itself is ~ormed.
The invention will be more fully understood from the following description of a preferred embodiment o~ a tubular collector element to which a ~hree-layer solar selective surface coating is applied. The description is yiven with reference to the accompanying drawings in which:
BRIEF DESCRIPTION OF T~E DRAWINGS
In the drawings:
Figure 1 shows a sectional elevation view of a glass collector element, Figure 2 shows a magnified view of a portion of the surface coating which is applied to the collector element, and ~S~5 Figure 3 shows a family of curves which ~lot calculated (infra-red) emittance of the surface coating against thickness of an intermediate layer of the surface coating.
DETAILED DESCRIPTI~N OF THE INVENTION
As illustrated in Figure 1, the tubular collector element lO comprises an inner (single-enaed) glass ~ube 11 and an outer glass tube 12. The outer tu~e is joined (i.e., welded) to the open end of the inner tube in a manner &uch that the outer surface of the inner tube is snveloped by the outer tube~ and the space 13 between the two tubes is subsequently evacuated.
The solar selective surf~ce coatiny, which i6 indicated by the dash-dotted line 14 in F'igure 1, is deposited on the outer surface of the inner tube prior to the end-joining of the two tubes. The surface coatin~ 1~ is deposited as three discrete layers and, as shown in Figure 2 the surface coating comprises:
(a) An inner layer 15 of copper which is deposited by a sputtering process to a thickness tl of about O.lO x 1O~6m., (b) An outer layer 16 of a semiconductor material which is ~eposited ~y a reactive sputtering process to a thickness t2 of Z x lO 6m., and (c) An in~ermediate layer 17 of a dielec~ric material which is reactively sputtered onto the inner layer to a ~hic~ness t3 of 0.5 x lO 6m.
~54~65 The respective layer materials have complex refractive indices:
Nl = nl-ikl ~or the copper layer 15, N2 = n2-ik2 for ~he semiconductor layer 16, and N3 = n3-ik3 for the (intermediate) dielectric material layer 17 and the materials are selected to satisEy ~he following relationship:
r~n3-ik3)-(nl-ikl)l2 > r(n2-ik~-(nl-ikl)l2 L(n3-ik3)*(nl-ikl)~
for normal inciden~ radia~ion over at least a major portion of the infra-red spectral range.
Suitable semiconductor and dielectric materials are silicon-germanium alloy and magnesium fluoride res~ectively.
Figure 3 ~hows a ~amily of analy~ically derived curves (A to D) ~hich plot hemispherical emi~tance EH f lnfra-red radiation against thickness of ~he dielectric tintermediate) layer 17 in the coa~ing of Figure 2 for diferent opera~ing tem~eratures. Curves A and B of Figure 3 relate to t~e emittance of coatings which are su~jected to an operating temperature of 300C, and curves B and C
relate ~o ~he emittance of coatings which are subjectad to an vperating temperature of 700 C. Curves A and C are applicable to a semiconductor material which has a relati~ely h;gh refractive index (n2~ 6), and curves B and are appliGable to a semiconductor material which has a relati~ely low refractive index (n2~ 3).
i2S9~5 It can be seen from these curves that the inclusion of the dielec~ric layer 17 contributes significan~ly to a reduction in the emittance of the coating and that ~enefits are to be derived from deposi~ing the dielectric layer to a thickness of at least 0.25 ~ 10 6m. Particularly beneficial results are obtained from depositing the dielectric layer ~o a thickness t3 in the order of 0.5 x 10~6m. to 1.5 x 10 6m.
FIELD OF THE INVENTION
This invention relates ~o a solar selective surface coating for absorber surfaces which are employed in solar collector~. In ~articular. the invention is directed to a solar selective surface coating which includes a refractive index mismatch layer for the purpose of reducing in~ra-red emittance.
BACKGRO~ND OF THE INVENTION
Solar æelective surface coatings known in the art usually comprise an ou~er layer of a material which absorbs strongly in the solar energy spectral range but which is transparent to infra-red radiation. In the interest of reducing ther~al losses, the outer layer is deposited on an inner layer of a material which provide6 hiyh reflectivity and, hence. low emissivity in the infra-red spectrum. A
typical such selective surface coating is disclosed in U.S.
Patent No.4.339,484, the surface coating comprising a metal-carbide solar energy absorp~ive outer layer and a copper infra red reflective base coating. The metal-carbide absorpti~e layer is graded such that it has a high carbide-to-metal ratio near its outer surface and a high metal-to carbide ratio adjacent the interface between the absorptive surface and the reflec~ive base coating.
A problem that is inherent in the traditional selective surface coatings is that the emissivi~y o~ the total sur~ace coating is much higher than the emissivity of the reflective base layer alone. One of the major reasons for this is that 4~i5 the emissivity o~ a material increases if the optical constan~s of a superimposed medium are different from those of free space, and this normally is the case with materials tha~ are employed for absorptive surface coatings. Such ma~erials, if semiconductors. may have refractive indices in the order o~ 2 to lO times greater than that of free space and, there~ore, the coupling of infra-r~d radiation across the metal-semiconductor interface may be significantly greater than tha~ of a metal-air interface.
SUMMARY OF THE INVENTION
The present invention seeks to alleviate this problem by providing a solar selective surface coating which comprises at least three layers: an inner layer composed of a material having high reflectivity and, hence, low emissivity in the infra-red spectral range, an outer layer composed o~ a material which i~ absorptive of energy in the solar energy spectral range and which is substantially ~ransparent to infra-red radiation, and an intermediate layer composed oP a material which i& substantially transparent to in~ra-red radiation. The inner, outer and intermediate layer materials ha~e complex refractive indices (nl-ikl), (n2-ik2~ and (n3-ik3) respectively and the materials are selected to satisfy the relationship r(n3-ik3)-(nl-ikl)l2 r~n2-ik2)-(nl-ikl)l2 L(n3_ik~)+(n~ tn2-ik2)~(nl-ikl~
for normal incident radiation over at least a major portion 6~
of the infra-red spectral range.
With ~he above defined surface coating structure, the re~ractive index mismatch between the inn~er layer and the intermediate layer is greater than it would be between the inner layer and the outer layer (in the absence of the intermediate layer) and, therefore, less infra-red radiation will be coupled ou~ of the inner layer. Stated in an alternative way, a higher value o infra-red reflectance i~
obtainea at the interface of the inner and intermediate layers than would be attainable at the interface of a two-layer solar selective surface coating.
PREFERRED FEATURES OP THE IMVENTION
The intermediate layer preferably is composea of a dielectric ma~erial which is highly transparent to infra-red radiation (i.e., having k3 approximately equal to 0) and which has a real refractive index n3 less than about 2.5.
Suitable such materials include magnesium fluoride, magnesium oxide, titanium oxide, aluminium oxide, silica, quartz and carbon.
The outer layer preferably i6 composed of a m~erial which beha~es as a semiconductor at collector operating temperatures. Such material may ha~e a monocrystalline, polycrystalline or amorphous structure and may comprise, ~or example, germanium, a germanium-sillcon alloy, silicon carbide, lead sulphide, boron or, in the case of surface coatings to be employed in relatively low ~emperatu~e collector systems, tellurium compounds. The material may ~25~ 5 alternatively be in the form of a cermet and, in such case, the mateLial forming the dielectric matrix of the cermet may be the same as or different from the dielectric material which forms the intermediate layer of the surface coating.
Also, the outer layer preferably is graded, either geome~rically or in ~erms of its composi~ion, such that its refractive index to solar radiation increases with increasing depth of ~he layer and such that the maximum ~real and imaginery) refractive indices (n2 and k2~
occur adjacent the interface with the intermediate layer.
Geometrical grading (i.e. texturing) may be achie~ed by chemically etching the outer layer o~ the surface coating.
Furthermore, the outer layer may be constituted by a number of sub-layers and include interference layers for the ~urpose of providing destructive interference to solar radiation. Simila~ly, the intermedia~e layer may include a nu~ber of sub-layers, but preliminary investigations tend to indica~e that no benefit is to be gained from s~ch a structure. ~owever, the point to be noted is that, allowing for the possibility of sub-layers, the total surface structure may, in ~act, include more than three layers.
The inner layer preferably c~mprises a metal, such as copper, aluminium, molybdenum, silver or gold which exhibits a high re~lec~ance to infra-red radiation, and such metal layer, when composed of copper, would normally be deposited to a thickness of at least O.lO x l~ 6m.
The intermediate layer preferably is deposited to a ~2Si4~
minimum thickness of 0.15 x 10 6m., and the outer layer would normally be deposited to produce a solar energy absorptiYe layer having a ~hickness in the order of 0.3 x 10~6m. to 5.0 x 10~6m.
The inner, intermediate and outer layer materials which are employed in any given coating de~irably should have coefficients of thermal expan6ion which are approximately equal, in order that the risk of differecltial ~ovement at the interface betwee~ ~he layers might be minimi6ed.
APPLICATION OF T~E INV~NTION
The solar selective surface coating may be applied to a flat plate-type collector surface. However, the surface coating would normally be applied to a tubular-type collector element having an inner (single or double ended) tube through which a heat exchange medium is caused to flow, an outer glass tube which envelopes the inner tube and an evacuated space between the two tubes. In the case of a tubular-type collector element, the surface coating would be deposited on the outer surface of the inner tube.
The inner tube may be formed from glass or metal.
depending upon the intended operating temperature of the collector system. Glass would normally be employed for operating temperature~ up to about 300C and metal for tempera~ures exceeding 300 C.
When a metal tube is employed, the tube itself may constitute the inner layer of the surface coating and, in such case, the intermediate layer would be deposited directly onto the outer surface of the metal tube. However, ~25~i5 .
a stainless steel or titanium tube would normally be used and, due to the relatively high infra-red emittance of such metals~ an inner layer of a low emi~tance metal would normally be coated onto the tube to form the inner layer.
Various techniques may be employed for aepssiting the respective layers of the surface coating onto the collector tube. For exam~le, electron beam, magnetron spu~tering, r.f. sputtering or, when appropriate, reac~ive sputtering deposition ~echniques may be employed for all layers.
Alternatively, the inner layer may be applied by electroplating, dipping or vapour deposition techniques, whilst the intermediate layer may be applied by dipping and the outer layer may be applied by chemical vapour deposition, The technique employed will depend on the ma~erial used in the respective layers and the material from which the collector tube itself is ~ormed.
The invention will be more fully understood from the following description of a preferred embodiment o~ a tubular collector element to which a ~hree-layer solar selective surface coating is applied. The description is yiven with reference to the accompanying drawings in which:
BRIEF DESCRIPTION OF T~E DRAWINGS
In the drawings:
Figure 1 shows a sectional elevation view of a glass collector element, Figure 2 shows a magnified view of a portion of the surface coating which is applied to the collector element, and ~S~5 Figure 3 shows a family of curves which ~lot calculated (infra-red) emittance of the surface coating against thickness of an intermediate layer of the surface coating.
DETAILED DESCRIPTI~N OF THE INVENTION
As illustrated in Figure 1, the tubular collector element lO comprises an inner (single-enaed) glass ~ube 11 and an outer glass tube 12. The outer tu~e is joined (i.e., welded) to the open end of the inner tube in a manner &uch that the outer surface of the inner tube is snveloped by the outer tube~ and the space 13 between the two tubes is subsequently evacuated.
The solar selective surf~ce coatiny, which i6 indicated by the dash-dotted line 14 in F'igure 1, is deposited on the outer surface of the inner tube prior to the end-joining of the two tubes. The surface coatin~ 1~ is deposited as three discrete layers and, as shown in Figure 2 the surface coating comprises:
(a) An inner layer 15 of copper which is deposited by a sputtering process to a thickness tl of about O.lO x 1O~6m., (b) An outer layer 16 of a semiconductor material which is ~eposited ~y a reactive sputtering process to a thickness t2 of Z x lO 6m., and (c) An in~ermediate layer 17 of a dielec~ric material which is reactively sputtered onto the inner layer to a ~hic~ness t3 of 0.5 x lO 6m.
~54~65 The respective layer materials have complex refractive indices:
Nl = nl-ikl ~or the copper layer 15, N2 = n2-ik2 for ~he semiconductor layer 16, and N3 = n3-ik3 for the (intermediate) dielectric material layer 17 and the materials are selected to satisEy ~he following relationship:
r~n3-ik3)-(nl-ikl)l2 > r(n2-ik~-(nl-ikl)l2 L(n3-ik3)*(nl-ikl)~
for normal inciden~ radia~ion over at least a major portion of the infra-red spectral range.
Suitable semiconductor and dielectric materials are silicon-germanium alloy and magnesium fluoride res~ectively.
Figure 3 ~hows a ~amily of analy~ically derived curves (A to D) ~hich plot hemispherical emi~tance EH f lnfra-red radiation against thickness of ~he dielectric tintermediate) layer 17 in the coa~ing of Figure 2 for diferent opera~ing tem~eratures. Curves A and B of Figure 3 relate to t~e emittance of coatings which are su~jected to an operating temperature of 300C, and curves B and C
relate ~o ~he emittance of coatings which are subjectad to an vperating temperature of 700 C. Curves A and C are applicable to a semiconductor material which has a relati~ely h;gh refractive index (n2~ 6), and curves B and are appliGable to a semiconductor material which has a relati~ely low refractive index (n2~ 3).
i2S9~5 It can be seen from these curves that the inclusion of the dielec~ric layer 17 contributes significan~ly to a reduction in the emittance of the coating and that ~enefits are to be derived from deposi~ing the dielectric layer to a thickness of at least 0.25 ~ 10 6m. Particularly beneficial results are obtained from depositing the dielectric layer ~o a thickness t3 in the order of 0.5 x 10~6m. to 1.5 x 10 6m.
Claims (14)
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A solar energy collector absorber having a solar selective surface comprising at least three superimposed layers including an inner layer composed of a material having high reflectivity in the infra-red spectral range, an outer layer composed of a semiconductor material which is absorptive of energy in the solar energy spectral range and which is substantially transparent to infra-red radiation, and an intermediate layer composed of a dielectric material which is substantially transparent to infra-red radiation, the outer layer material being deposited to a thickness not less than 0.3 x 10-6m., the intermediate layer material being deposited to a thickness within the range of about 0.15 to 2.0 x 10-6, the inner, outer and intermediate layer materials having complex refractive indices (n1-ik1), (n2-ik2) and (n3-ik3) respectively and the materials being selected to satisfy the relationship for normal incident radiation over at least a major portion of the infra-red spectral range.
2. A solar energy collector absorber as claimed in claim 1 wherein said inner layer is deposited onto a surface of the absorber.
3. A solar energy collector absorber as claimed in claim 1 when in the form of a tube on which the solar selective surface is deposited.
4. A collector element for use in a solar collector system and which comprises an inner tube through which fluid can be passed, an outer glass tube enveloping at least a portion of the periphery of the inner tube and defining an evacuated space between the two tubes, and a solar selective surface coating deposited on the outer surface of the inner tube;
the solar selective surface coating comprising at least three layers including an inner layer composed of a material having high reflectivity in the infra-red spectral range, an outer layer composed of a semiconductor material which is absorptive of energy in the solar energy spectral range and which is substantially transparent to infra-red radiation, and an intermediate layer composed of a dielectric material which is substantially transparent to infra-red radiation, the outer layer material being deposited to a thickness not less than 0.3 x 10-6m., the intermediate layer material being deposited to a thickness within the range of about 0.15 to 2.0 x 10-6m., the inner, outer and intermediate layer materials having complex refractive indices (n1-ik1), (n2-ik2) and (n3-ik3) respectively and the materials being selected to satisfy the relationship for normal incident radiation over at least a major portion of the infra-red spectral range.
the solar selective surface coating comprising at least three layers including an inner layer composed of a material having high reflectivity in the infra-red spectral range, an outer layer composed of a semiconductor material which is absorptive of energy in the solar energy spectral range and which is substantially transparent to infra-red radiation, and an intermediate layer composed of a dielectric material which is substantially transparent to infra-red radiation, the outer layer material being deposited to a thickness not less than 0.3 x 10-6m., the intermediate layer material being deposited to a thickness within the range of about 0.15 to 2.0 x 10-6m., the inner, outer and intermediate layer materials having complex refractive indices (n1-ik1), (n2-ik2) and (n3-ik3) respectively and the materials being selected to satisfy the relationship for normal incident radiation over at least a major portion of the infra-red spectral range.
5. A collector element as claimed in claim 4 wherein the inner tube is a glass tube.
6. A collector element as claimed in claim 4 wherein the inner tube is a metal tube.
7. A collector element as claimed in claim 4 wherein the inner layer is composed of a metal selected from the group of copper, aluminium, molybdenum, silver and gold and wherein the inner layer is deposited to a thickness not less than 0.10 x 10-6m.
8. A collector element as claimed in claim 4 wherein the intermediate layer is composed of a material selected from the group magnesium fluoride, magnesium oxide, titanium oxide, aluminium oxide, silica, quartz and carbon.
9. A collector element as claimed in claim 4 wherein the outer layer is composed of a material selected from the group germanium, germanium-silicon alloy, silicon carbide, lead sulphide and boron.
10. A collector element as claimed in claim 4 wherein the outer layer is composed of a cermet.
11. A collector element as claimed in claim 10 wherein the intermediate layer is composed of a dielectric material which has the same composition as a material forming the dielectric matrix of the cermet.
12. A collector element as claimed in claim 4 wherein the outer layer has a thickness falling within the range 0.3 x 10-6m. to 5.0 x 10-6m.
13. A collector element as claimed in claim 4 wherein the outer layer is graded as to its composition such that the refractive index to solar radiation increases with increasing depth of the layer.
14. A collector element as claimed in claim 4 wherein the outer layer is graded geometrically by etching the outermost surface of the coating whereby the refractive index to solar radiation increases with increasing depth of the layer.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPF6254 | 1982-10-08 | ||
| AUPF625482 | 1982-10-08 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1254465A true CA1254465A (en) | 1989-05-23 |
Family
ID=3769783
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000438343A Expired CA1254465A (en) | 1982-10-08 | 1983-10-04 | Solar selective surface coating |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4628905A (en) |
| EP (1) | EP0107412B1 (en) |
| JP (1) | JPS5989956A (en) |
| AT (1) | ATE31361T1 (en) |
| CA (1) | CA1254465A (en) |
| DE (1) | DE3374899D1 (en) |
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| JPS61255556A (en) * | 1985-05-07 | 1986-11-13 | Aisin Seiki Co Ltd | Holding and retrieving device for information medium |
| CA2039109A1 (en) * | 1990-04-23 | 1991-10-24 | David B. Chang | Selective emissivity coatings for interior temperature reduction of an enclosure |
| AU646172B2 (en) * | 1991-07-19 | 1994-02-10 | Universal Solar and Surface Science Proprietary Limited | Thin film solar selective surface coating |
| IN185567B (en) * | 1991-07-19 | 2001-03-03 | Univ Sydney | |
| DE4234967A1 (en) * | 1992-10-16 | 1994-04-21 | Schmidt Manfred Prof Dr Ing Ha | Solar collector - Has several porous layers of different spectral transmissivity arranged in hollow absorber, through which energy-carrying medium flows |
| DE4433863A1 (en) * | 1994-09-22 | 1996-03-28 | Interpane Entw & Beratungsges | Spectral selective collector coating and process for its production |
| JPH08118842A (en) * | 1994-10-18 | 1996-05-14 | Houyuu Shuppan Kk | Book cover |
| WO1998026926A1 (en) * | 1996-12-19 | 1998-06-25 | Energy Conversion Devices, Inc. | Selective solar radiation control coatings for windows and plastic films characterized by an absence of silver |
| DE20021644U1 (en) * | 2000-12-20 | 2002-05-02 | Alanod Al Veredlung Gmbh | Solar collector element |
| DE10150452C1 (en) * | 2001-10-16 | 2003-05-22 | Schott Glas | Process for the production of a vitreous body and use of the vitreous body |
| DE102004038233A1 (en) * | 2004-08-05 | 2006-03-16 | Schott Ag | solar absorber |
| SE530464C2 (en) * | 2005-08-02 | 2008-06-17 | Sunstrip Ab | Nickel alumina coated solar absorber |
| US8378280B2 (en) | 2007-06-06 | 2013-02-19 | Areva Solar, Inc. | Integrated solar energy receiver-storage unit |
| CN101984761A (en) * | 2007-06-06 | 2011-03-09 | 奥斯拉公司 | Combined cycle power plant |
| US9022020B2 (en) | 2007-08-27 | 2015-05-05 | Areva Solar, Inc. | Linear Fresnel solar arrays and drives therefor |
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| JP5331131B2 (en) * | 2008-02-20 | 2013-10-30 | コーニング インコーポレイテッド | Solar thermal collector with glass ceramic center pipe |
| ES2316321B2 (en) | 2008-10-20 | 2010-12-14 | Abengoa Solar New Technologies, S.A. | SOLAR SELECTIVE ABSORBENT COATING AND MANUFACTURING METHOD. |
| ITMI20090216A1 (en) * | 2009-02-18 | 2010-08-19 | Azzimonti Paolino Spa | MANUFACTURED IN GLASS, INCLUDING SILVERING AND COUCHE LOW EMISSIVES FOR THE TRANSFORMATION OF SOLAR IRRADIATION IN REUSABLE ENERGY |
| DE102009049471B3 (en) * | 2009-10-15 | 2011-04-07 | Schott Solar Ag | Radiation-selective absorber coating and absorber tube with radiation-selective absorber coating |
| CN101737983B (en) * | 2009-11-25 | 2012-12-12 | 北京航空航天大学 | Solar spectrum selective absorbing coating and preparation method thereof |
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| CN103185405A (en) * | 2011-12-31 | 2013-07-03 | 成都首能新能源开发有限公司 | Vacuum glass assembly with condensing lens array |
| EP2834574B1 (en) * | 2012-01-05 | 2020-08-26 | Norwich Technologies, Inc. | Linear Solar Receiver for Concentrating Solar Power Systems |
| JP2015111011A (en) * | 2012-03-23 | 2015-06-18 | 旭硝子株式会社 | Light selective absorption film, heat collection pipe, and solar heat power generation system |
| JP6059952B2 (en) * | 2012-10-26 | 2017-01-11 | 株式会社豊田自動織機 | Heat conversion member and heat conversion laminate |
| JP5994569B2 (en) * | 2012-10-26 | 2016-09-21 | 株式会社豊田自動織機 | Heat conversion member and heat conversion laminate |
| JP2015049015A (en) * | 2013-09-04 | 2015-03-16 | 日立造船株式会社 | Collector |
| US10215447B2 (en) * | 2015-04-29 | 2019-02-26 | California Institute Of Technology | Spectrally selective semiconductor dielectric photonic solar thermal absorber |
| WO2019195637A1 (en) | 2018-04-04 | 2019-10-10 | The Research Foundation For The State University Of New York | Systems and methods for passive cooling and radiator for same |
| CN112443991B (en) * | 2019-08-28 | 2022-06-17 | 香港科技大学 | Selective solar energy absorbing coating processed based on solution method and preparation method thereof |
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| US3176678A (en) * | 1963-04-02 | 1965-04-06 | Engelhard Ind Inc | Solar energy collector |
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| US4029853A (en) * | 1975-06-20 | 1977-06-14 | The United States Of America As Represented By The Secretary Of The Navy | PbS-Al selective solar absorber |
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| US4361630A (en) * | 1979-04-20 | 1982-11-30 | The United States Of America As Represented By The Secretary Of The Commerce | Ultra-black coating due to surface morphology |
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| US4442166A (en) * | 1979-11-15 | 1984-04-10 | Massachusetts Institute Of Technology | Cermet film selective-black absorber |
| JPS5911827B2 (en) * | 1979-12-27 | 1984-03-17 | シャープ株式会社 | Selective absorption surface of heat collection part |
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| WO1982000510A1 (en) * | 1980-07-28 | 1982-02-18 | Spanoudis L | Solar energy collector having semiconductive coating formed from metal and dielectric |
| US4356815A (en) * | 1980-08-19 | 1982-11-02 | Owens-Illinois, Inc. | Solar energy collector having an absorber element of coated foil |
| US4321300A (en) * | 1980-11-12 | 1982-03-23 | Engelhard Minerals & Chemicals Corp. | Thin film solar energy collector |
| US4416916A (en) * | 1982-03-04 | 1983-11-22 | Engelhard Corporation | Thin film solar energy collector |
| US4437455A (en) * | 1982-05-03 | 1984-03-20 | Engelhard Corporation | Stabilization of solar films against hi temperature deactivation |
-
1983
- 1983-09-30 EP EP83305961A patent/EP0107412B1/en not_active Expired
- 1983-09-30 DE DE8383305961T patent/DE3374899D1/en not_active Expired
- 1983-09-30 AT AT83305961T patent/ATE31361T1/en not_active IP Right Cessation
- 1983-10-04 CA CA000438343A patent/CA1254465A/en not_active Expired
- 1983-10-08 JP JP58189145A patent/JPS5989956A/en active Granted
-
1985
- 1985-04-23 US US06/726,180 patent/US4628905A/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| US4628905A (en) | 1986-12-16 |
| JPS5989956A (en) | 1984-05-24 |
| EP0107412B1 (en) | 1987-12-09 |
| ATE31361T1 (en) | 1987-12-15 |
| EP0107412A1 (en) | 1984-05-02 |
| JPH0222863B2 (en) | 1990-05-22 |
| DE3374899D1 (en) | 1988-01-21 |
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