Absorbent material and solar panel using such a material

Abstract

The invention concerns a multilayer material comprising at least: a support having a reflectance R higher than 80% for radiations of wavelengths higher than 5 m, a selective layer comprising a combination of Vanadium oxides VO.sub.2 and VO.sub.2O.sub.2n+/1, with n>1, said selective layer having an absorbance higher than 75% for radiations of wavelengths comprised between 0.4 and 2.5 m, regardless of the temperature T, and having, for radiations of wavelengths comprised between 6 and 10 m, a transmittance Tr such that: Tr>85% for T<Tc, a critical temperature, 20%Tr50% for T>Tc. Application to the production of thermal solar panels having a low stagnation temperature and high performance.

Claims

1. A multilayer material comprising at least: a support having a reflectivity R higher than 80% for radiations of wavelengths higher than 5 m, a selective layer having a thickness comprised between 100 and 500 nm, said selective layer comprising a combination of vanadium oxides VO.sub.2 and V.sub.nO.sub.2n+/1, with n>1, said selective layer having an absorbance higher than 75% for radiations of wavelengths comprised between 0.4 and 2.5 m, regardless of the temperature T, and having, for radiations of wavelengths comprised between 6 and 10 m, a transmittance Tr such that: Tr>85% for T<Tc, a critical temperature, 20%Tr50% for T>Tc, wherein, for radiations of wavelengths comprised between 6 and 10 m, the support has an optical index n1 and the selective layer has an optical index n2 such that: n2<n1 regardless of the temperature T, and n2<6 T>Tc.

2. Material according to claim 1, wherein the selective layer has an extinction coefficient k lower than 4 for radiations of wavelengths comprised between 6 and 10 m.

3. Material according to claim 1, wherein, for radiations of wavelengths between 6 and 10 m, the optical index n2 is comprised between 0.8*(n1).sup.1/2 and 1.2*(n1).sup.1/2 for T>Tc.

4. Material according to claim 1, wherein the selective layer has a thickness comprised between 100 and 200 nm.

5. Material according to claim 1, wherein the selective layer is doped with at least one metal M different from Vanadium.

6. Material according to claim 5, wherein the selective layer is doped with aluminum and has a critical temperature comprised between 80 C. and 120 C.

7. Material according to claim 5, wherein the selective layer has a concentration in the dopant M sufficient to form at least one oxide of the form M.sub.1-xO.sub.x, with 0<x<1, x being the atomic fraction of oxygen in the oxide, so that the selective layer comprises a combination of oxides of the type VO.sub.2, V.sub.nO.sub.2n+/1, and M.sub.1-xO.sub.x.

8. Material according to claim 7, wherein the oxide or oxides in the form M.sub.1-xO.sub.x have a transmittance higher than 85% for infrared radiations whose wavelengths are comprised between 6 and 10 m.

9. Material according to claim 8, wherein the oxide of the form M.sub.1-xO.sub.x is an aluminum oxide.

10. Material according to claim 1, wherein the selective layer is covered with an antireflection layer having, for radiations whose wavelengths are comprised between 0.4 and 2.5 m, an optical index n3<n2, n2 being the optical index of the selective layer.

11. Material according to claim 10, wherein the antireflective layer has a thickness comprised between 10 and 150 nm.

12. Material according to claim 1, further comprising, between the selective layer and the support, an adhesive layer, for example, a metal layer, an oxide layer, a layer of transition metal nitrides, or a layer of a mixture of these materials, having a thickness comprised between 5 and 100 nm.

13. Material according to claim 1, wherein the selective layer comprises: a combination of VO.sub.2 and V.sub.4O.sub.9 Vanadium oxides, or a combination of VO.sub.2 and V.sub.6O.sub.13 Vanadium oxides, or a combination of VO.sub.2 and V.sub.4O.sub.9 Vanadium oxides and Al.sub.2O.sub.3 oxide.

14. Solar panel comprising a multilayer material according to claim 1.

15. Material according to claim 2, wherein, for radiations of wavelengths between 6 and 10 m, the optical index n2 is comprised between 0.8*(n1).sup.1/2 and 1.2*(n1).sup.1/2 for T>Tc.

16. Material according to claim 5, wherein the at least one metal M is aluminum, chromium, or titanium.

17. Material according to claim 8, wherein the oxide of the form M.sub.1-xO.sub.x is Al.sub.2O.sub.3.

18. Material according to claim 8, wherein the oxide of the form M.sub.1-xO.sub.x is an under-stoichiometric aluminum oxide.

19. A multilayer material comprising at least: a support having a reflectivity R higher than 80% for radiations of wavelengths higher than 5 m, a selective layer having a thickness comprised between 100 and 500 nm, said selective layer comprising a combination of vanadium oxides VO.sub.2 and V.sub.nO.sub.2n+/1, with n>1, said selective layer having an absorbance higher than 75% for radiations of wavelengths comprised between 0.4 and 2.5 m, regardless of the temperature T, and having, for radiations of wavelengths comprised between 6 and 10 m, a transmittance Tr such that: Tr>85% for T<Tc, a critical temperature, 20%Tr50% for T>Tc, wherein the selective layer is covered with an antireflection layer having, for radiations whose wavelengths are comprised between 0.4 and 2.5 m, an optical index n3<n2, n2 being the optical index of the selective layer.

20. Material according to claim 19, wherein the antireflective layer has a thickness comprised between 10 and 150 nm.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will be better understood, and other features and advantages of the invention will appear, in light of the following description of examples of materials according to the invention. These examples are given as non-limiting examples. The description is to be read in conjunction with the accompanying drawings in which

(2) FIG. 1 details the constitution of a multilayer material comprising a selective layer based on pure VO.sub.2,

(3) FIGS. 2-4 show the constitution of multi-layer materials according to the invention,

(4) FIG. 5 shows an X-ray diffraction pattern of the layer 32,

(5) FIG. 6 shows results of measurements made on the material of FIG. 3, and

(6) FIG. 7 shows results of measurements made on the material of FIG. 4.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

(7) FIG. 1 shows a material 10 comprising a support 11 covered with a layer 12 of pure vanadium oxide VO.sub.2. The layer 12 has thermochromic properties and in particular transmittance in the infrared range (wavelengths of between 6 and 10 m) Tr lower than 10% at T>Tc. The layer 12 is thus almost opaque to infrared radiation at high temperature. The layer 12 has a thickness in the order of 100 to 500 nm.

(8) An example of material 20 according to the invention is shown in FIG. 2. It comprises a support 21 and a layer 22 of two-phase material comprising vanadium oxides of the type VO.sub.2 and V.sub.4O.sub.9, possibly doped with a dopant concentration lower than the critical concentration. The layer 22 has a thickness in the order of 100 to 500 nm.

(9) The supports 11, 21 are made in a material that is opaque and reflects infrareds (reflectance R greater than 80% for radiations of wavelengths greater than 5 m), and mechanically sufficiently resistant in order to be able to produce a rigid plate of large dimensions (1 to 3 m.sup.2 surface area). They are for example made in aluminum (reflectance R>90%), or in a material having sufficient mechanical strength, covered with an opaque layer of aluminum. The optical index n1 and the extinction coefficient k1 of the support are respectively in the order of 5 to 25 and 30 to 86 for wavelengths comprised between 6 and 10 m.

(10) The layer 12 (VO.sub.2) has the following properties: solar absorbance in the order of 75-80% infrared transmittance Tr (wavelengths comprised between 6 and 10 m): in the order of 90% when the temperature T is lower than Tc in the order of 65 to 120 C. in the order of 5% when the temperature T is higher than Tc

(11) The layer 22 (VO.sub.2+V.sub.4O.sub.9) has the following properties: solar absorbance in the order of 75-80% infrared transmittance Tr (wavelength comprised between 6 and 10 m): in the order of 90% when the temperature T is lower than Tc in the order of 65 to 120 C. in the order of 25 to 35% when the temperature T is higher than Tc an optical index n2 lower than the optical index n1 of the substrate 21, i.e., in the order of 4 to 6 for wavelengths comprised between 6 and 10 m.

(12) For low temperatures (T<Tc), the layer 12 is quasi transparent to infrared radiation (transmittance in the order of 90%); thus, the emissivity of the material 10 depends essentially on the emissivity of the support 11; the support 11 being a reflector of infrareds, its infrared emissivity is very low so that the infrared emissivity of the material 10 is very low; therefore, the material 10 has very low thermal losses for temperatures below Tc. In addition, the material 10 has an absorbance equal to that of the layer 12, in the order of 75 to 80%, and thus, a high conversion efficiency of solar energy. The material 20 has the same behavior as the material 10 for T<Tc.

(13) For high temperatures (T>Tc), the layer 12 is quasi opaque to infrared radiation (transmittance lower than 10%); Thus, the emissivity of the material 10 depends essentially on the emissivity of the layer 12, in the order of 25 to 30%. The lowest stagnation temperature of the material 10 is about 180 C.

(14) In contrast, for high temperatures (T>Tc), the layer 22 is partially transparent to infrared radiation (transmittance in the order of 30%); the infrared reflectance of the material 20 is therefore less than that of the material 10 and its infrared emissivity is greater than that of the material 10; in addition, the increase in the path of the infrared radiation in the layer 22 makes it possible to increase the infrared emissivity more at T>Tc. Finally, n2 being lower than n1, the layer 22 can play an antireflection role on the support 21 and reduce the optical reflection of the multilayer material 20 more. In these conditions, the emissivity of the material 20 is higher than 35%, preferably higher than 40%. The material 20 thus heats up much less than the material 10. Tests have shown that the stagnation temperature of the material 20 is, in practice, in the order of 140 to 160 C., and in all cases lower than 170 C.

(15) FIG. 3 is another example of a material 30 according to the invention. The material 30 comprises a support 31 on which is deposited a selective layer 32 comprising a combination of oxides VO.sub.2 and V.sub.4O.sub.9, possibly doped with a dopant concentration lower than the critical concentration. FIG. 5 shows by X-ray diffraction the presence of the VO.sub.2 and V.sub.4O.sub.9 phases with an important proportion of the V.sub.4O.sub.9 phase. The layer 32 thus has a partial transmittance comprised between 25 and 30% for wavelengths comprised between 6 and 10 m. The material 30 comprises, as a complement, an antireflection layer 33 produced in a material such as silicon oxide SiO.sub.2, characterized by an optical index n3 close to 1.5 for wavelengths comprised between 0.3 and 2.5 m. The antireflection layer 33 makes it possible to improve the absorbance of the material 30, which thus reaches more than 90%. The thickness of the layer 33 is adjusted to maximize the antireflection effect in the visible range, which corresponds approximately to the maximum solar emission; to this effect, the thickness of the layer 33 is chosen close to a quarter of the wavelength of the visible radiation divided by n3, i.e., a thickness in the order of 60 nm (380 nm/4/1.5=63 nm), at 130 nm (780 nm/4/1.5=130 nm), for visible radiation at a wavelength comprised between 380 nm and 780 nm.

(16) FIG. 6 shows the variation of the reflectance R (reflectance at T>Tc less reflectance at T<Tc), and thus, of the emissivity, as a function of the wavelength, for: the material 30: Aluminium support/VO.sub.2+V.sub.4O.sub.9 absorbent layer/SiO.sub.2 layer the material 10 with an antireflection layer of the same nature and of the same thickness as that deposited on the material 30: Aluminium support/VO.sub.2+V.sub.4O.sub.9 absorbent layer/SiO.sub.2 layer
It is seen that in the case of the material 30, the variation of the emissivity R.sub.30 is greater than the variation of the emissivity R.sub.10 of the material 10. This important variation results in a significant decrease of the stagnation temperature. In addition, and as explained in the description of the invention, the multilayer material according to the invention also has an infrared reflectance at T<Tc greater that a multilayer material in which the selective layer consists of pure VO.sub.2. In these conditions, the performance at T<Tc of a solar panel equipped with the multilayer material according to the invention is increased.

(17) FIG. 4 is another example of a material 40 according to the invention. The material 40 comprises a support 41 on which is deposited a selective layer 42 comprising a combination of oxides VO.sub.2, V.sub.4O.sub.9, and Al.sub.2O.sub.3 (obtained by doping with an aluminum atomic concentration of 10%, i.e., higher than the critical concentration), covered by an antireflection layer 43. The material 40 has a solar absorption between 0.4 and 2.5 m higher than 94% as well as an infrared emissivity measured at 8 m varying between 5 and 45% as a function of the temperature.

(18) FIG. 7 shows the variation of the reflectance R (reflectance at T>Tc less reflectance at T<Tc), and thus, of the emissivity, as a function of the wavelength, for the material 40, and shows a variation of the emissivity R.sub.40 that is improved even more as compared to the material 30. That is, whereas R.sub.30 is in the order of 38%, R.sub.40 reaches about 46% for a wavelength in the order of 8 m. FIG. 7 also shows a better behavior of the material 40 at low temperature with a reflection coefficient at 8 m in the order of 96% (i.e., emissivity close to 4%).

(19) In these conditions, a thermal solar cell equipped with the material 40 according to the invention operates, at T<Tc, with Tc>80 C., identically to a standard thermal solar cell, with a solar absorption of 94% and an infrared emissivity of 5%, and in addition, makes it possible, at T>Tc, to reduce considerably the stagnation temperature to a value lower than 160 C.