SOLAR THERMAL COLLECTION APPARATUS AND METHODS
A solar thermal collector includes a receptacle and a fluid conduit. The receptacle is evacuated to a subatmospheric pressure. The receptacle includes a window and a reflector facing the window. The window and the reflector are exposed to the subatmospheric pressure in the receptacle. The fluid conduit extends through the receptacle between the window and the reflector. The reflector concentrates solar radiation passing through the window onto the fluid conduit.
Solar thermal collectors collect heat by absorbing solar radiation. Solar thermal collectors come in a variety of different types, including flat-plate collectors, evacuated tubes, and solar concentrating collectors. A flat plate solar thermal collector has a broad flat plate solar radiation absorber, whereas an evacuated tube collector contains an absorber within an evacuated tube. A concentrating collector includes a reflector that focuses radiant energy onto a localized solar radiation absorber. In both cases, the solar radiation absorber converts the solar radiation into heat energy that typically is transferred to a circulating heat transfer fluid. Solar thermal collectors may be incorporated into stationary installations or into installations that track solar azimuthal position.
In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.
At least some of the examples that are described herein provide solar collection apparatus and methods that reduce manufacturing costs, improve reliability, and improve efficiency through the incorporation of a reflector and a fluid conduit into an evacuated receptacle. These examples eliminate the plurality of individual evacuated glass tubes that typically are used in prior solar panel designs to contain the absorber elements. In this way, these examples are able to achieve low convective losses while avoiding the increased manufacturing complexity and cost associated with such evacuated tube based solar thermal collectors. At least some of these examples may be assembled with substantially fewer fluid connections as compared to evacuated-tube-based solar thermal collectors, which require a fluid connection for each tube. By reducing the number of fluid connections and their associated convective losses, the operating performance of these examples are expected to be higher than evacuated tube based solar thermal collectors. In addition, by containing the fluid connections within the evacuated space, these examples enable convective losses and insulation requirements to be significantly reduced.
The reflector 18 may be any type of reflector that concentrates incoming solar radiation onto the fluid conduit 14, including imaging solar concentrators (e.g., cylindrical and parabolic concentrators) and non-imaging solar concentrators that use non-imaging optics geometries to concentrate solar radiation onto the fluid conduit 14. In the illustrated example, the reflector 18 includes a pair of concave radiation-reflective surface portions 20, 22 that meet along a longitudinal axis 24 (represented by the center of the dashed circle) in a longitudinal plane about which the concave radiation-reflective surface portions 20, 22 are symmetrical. In other examples, the concave radiation-reflective surface portions are asymmetrically offset on opposite sides of the longitudinal plane. In some examples, the pair of concave surface portions form a non-imaging concentrator collector (NICC) trough (see
In the illustrated embodiment, the reflector 18 is integrally incorporated into a base 26 of the receptacle 12. In some examples, the base 26 is a unitary metal structure that comprises the reflector 18. The unitary metal structure may be formed, for example, by a metal extrusion process (e.g., an aluminum extrusion process) or a metal stamping process (e.g., a steel stamping process). In other examples, the base 26 is an extruded or molded plastic piece and the reflector 18 is made of a metal (e.g., an aluminum or silver film) that is bonded to the plastic base. In some examples, a getter (e.g., a zirconium based getter) is applied to the reflector 18 in order to capture gases that outgas from or leakage into the receptacle 12 and thereby prevent oxidation of the receptacle (which can serve as a source of emissive loss) after the receptacle 12 has been sealed and evacuated.
The fluid conduit 14 extends adjacent the reflector 18 along the confocal solar concentration line 25 of the reflector 18. The fluid conduit 14 may be any type of structure through which a heat transfer fluid (e.g., air, water, oil such as synthetic paraffin oil, and super critical carbon dioxide) can be circulated. In some examples, the fluid conduit 14 is a hollow cylindrical metal tube 28 (e.g., an aluminum or copper tube) that includes an outer surface and an inner surface. In the illustrated example, the outer surface of the fluid conduit 14 carries a solar radiation absorbent coating 30. The coating 30 typically includes a solar selective absorbent layer (e.g., aluminum nitride, metal-aluminum nitride cermets, or titanium oxide layer) and, optionally, one or more additional layers, including an overlying antireflection layer that passes solar radiation to the radiation absorbent layer with minimal reflections and an underlying stabilizing layer under the radiation absorbent layer. In some examples, the solar radiation absorbent coating 30 targets maximum absorption of solar thermal energy and rejects solar spectra outside this range, while maintaining an ideal emissivity of less than 6% at 180° Celsius. In some examples, the inner surface of the fluid conduit 14 is textured with surface features that disturb the flow of fluid through the fluid conduit 14 to reduce laminar flow and increase heat transfer to the circulating fluid. In some of these examples, the textured inner surface of the fluid conduit 14 is formed by creating patterns (e.g., helical patterns) of ribs and grooves in the inner surface of the fluid conduit using a knurling tool.
Referring back to
In the illustrated example, the base 26 of the receptacle 12 includes a peripheral groove 38 into which the window 16 is recessed such that the bottom edges of the window 16 are supported by the foot of the groove and the top edges of the window are flush with the top surface of the base 26. The window 16 is mounted to the base 26 with a connection that maintains the subatmospheric pressure in the receptacle 12. In examples in which the base is formed of a metal (e.g., aluminum) and the window 16 is formed of glass, the connection between the window 16 and the base 26 may include a glass-to-metal seal (e.g., a solder based seal or a laser weldable glass or ceramic seal) that maintains the subatmospheric pressure in the receptacle 12.
In accordance with the method of
The window 16 is mounted to the base 26 to form the receptacle 12 containing the fluid conduit 14, where the reflector 18 faces the window 16 and concentrates solar radiation passing through the window onto the fluid conduit 14 (
The receptacle 12 is evacuated to a subatmospheric pressure, where the window 16 and the reflector 18 are exposed to the subatmospheric pressure in the receptacle 12 (
The reflector 48 includes an array of reflector elements 54, 56, 58, 60, 62, 64, 66, 68 each of which corresponds to the reflector 18 described above. In the illustrated example, each of the reflector elements 54-68 is formed from a respective pair of concave radiation-reflective surface portions, the concave radiation-reflective surface portions of each reflector element 54-68 meet along a respective longitudinal axis in a respective longitudinal plane about which the concave radiation-reflective surface portions are either symmetrically or asymmetrically disposed, and the respective longitudinal axes are parallel. Each reflector element 54-68 concentrates radiation passing through the window onto a different respective section of the fluid conduit 50.
Referring to
Referring back to
Table 1 below shows a comparison of the relative sizes of components of an exemplary evacuated-tube-based solar thermal collector and an exemplary tubeless solar thermal collector of the type shown in
In some examples, the solar panel array shown in
In accordance with the method of
Fluid is circulated through the fluid conduit (
Other embodiments are within the scope of the claims.
Claims
1. A solar thermal collector, comprising
- a receptacle evacuated to a subatmospheric pressure and comprising a window and a reflector facing the window, wherein the window and the reflector are exposed to the subatmospheric pressure in the receptacle; and
- a fluid conduit extending through the receptacle between the window and the reflector, wherein the reflector concentrates solar radiation passing through the window onto the fluid conduit.
2. The solar thermal collector of claim 1, wherein the reflector comprises a pair of concave radiation-reflective surface portions that meet along a longitudinal axis in a longitudinal plane.
3. The solar thermal collector of claim 2, wherein the fluid conduit extends adjacent the reflector along a direction in the longitudinal plane that is parallel to the longitudinal axis.
4. The solar thermal collector of claim 1, wherein the reflector comprises a plurality of reflector elements each comprising a respective pair of concave radiation-reflective surface portions, the concave radiation-reflective surface portions of each reflector element meet along a respective longitudinal axis in a respective longitudinal plane, and the respective longitudinal axes are parallel.
5. The solar thermal collector of claim 4, wherein the fluid conduit comprises parallel linear segments each of which extends adjacent a respective one of the reflector elements along a respective direction in a respective one of the longitudinal planes that is parallel to the respective longitudinal axis.
6. The solar thermal collector of claim 5, wherein the fluid conduit comprises curved segments that interconnect the parallel linear segments to define a serpentine fluid flow path adjacent the reflector.
7. The solar thermal collector of claim 4, wherein each reflector element concentrates radiation passing through the window onto different respective sections of the fluid conduit.
8. The solar thermal collector of claim 1, wherein the receptacle comprises a base, and the window is mounted to the base with a connection that maintains the subatmospheric pressure in the receptacle.
9. The solar thermal collector of claim 8, wherein the base integrally incorporates the reflector.
10. The solar thermal collector of claim 9, wherein the base is a unitary metal structure that comprises the reflector.
11. The solar thermal collector of claim 10, wherein the window is formed of glass, and further comprising between the window and the base a glass-to-metal seal that maintains the subatmospheric pressure in the receptacle.
12. The solar thermal collector of claim 9, wherein the base is plastic and the reflector is bonded to the plastic base.
13. The solar thermal collector of claim 1, wherein the window comprises an antireflective coating.
14. The solar thermal collector of claim 13, wherein the antireflective coating is porous.
15. The solar thermal collector of claim 1, wherein the window comprises first and second parallel surfaces each of which is coated with a respective antireflective coating.
16. The solar thermal collector of claim 1, wherein the fluid conduit comprises an outer surface carrying a radiation-absorbent coating, and an inner surface exposed for contact with fluid flowing through the fluid conduit.
17. The solar thermal collector of claim 1, wherein the fluid conduit comprises an outer surface, and a textured inner surface exposed for contact with fluid flowing through the fluid conduit.
18. The solar thermal collector of claim 1, wherein the fluid conduit comprises a plurality of fluid channels for conveying fluid.
19. The solar thermal collector of claim 18, wherein each of the fluid channels has a respective inner diameter between 0.75 millimeter and 0.25 millimeter.
20. A method of manufacturing a solar thermal collector, comprising
- attaching a fluid conduit to a base comprising a reflector;
- mounting a window to the base to form a receptacle containing the fluid conduit, wherein the reflector faces the window and concentrates solar radiation passing through the window onto the fluid conduit; and
- evacuating the receptacle to a subatmospheric pressure, wherein the window and the reflector are exposed to the subatmospheric pressure in the receptacle.
21. The method of claim 20, wherein the reflector comprises a plurality of reflector elements each comprising a respective pair of concave radiation-reflective surface portions, the concave radiation-reflective surface portions of each reflector element meet along a respective longitudinal axis in a respective longitudinal plane, and the respective longitudinal axes are parallel.
22. The method of claim 21, wherein each reflector element concentrates radiation passing through the window onto a different respective section of the fluid conduit.
23. The method of claim 20, wherein the receptacle comprises a base, and the mounting comprises attaching the window to the base with a connection that maintains the subatmospheric pressure in the receptacle.
24. The method of claim 20, wherein the base is a unitary metal structure that comprises the reflector, the window is formed of glass, and the mounting comprises forming between the window and the base a glass-to-metal seal that maintains the subatmospheric pressure in the receptacle.
25. The solar thermal collector of claim 20, wherein the fluid conduit comprises a plurality of fluid channels for conveying fluid.
26. The method of claim 20, further comprising purging the receptacle with an inert gas before evacuating the receptacle.
27. A solar collection method, comprising
- providing a solar thermal collector comprising a receptacle evacuated to a subatmospheric pressure and comprising a window and a reflector facing the window, wherein the window and the reflector are exposed to the subatmospheric pressure in the receptacle, and a fluid conduit extending through the receptacle between the window and the reflector, wherein the reflector concentrates solar radiation passing through the window onto the fluid conduit; and
- circulating fluid through the fluid conduit.
28. The method of claim 27, wherein the fluid comprises super-critical carbon dioxide.
29. The method of claim 28, wherein the fluid conduit comprises a plurality of fluid channels for conveying the fluid.
30. The method of claim 29, wherein each of the fluid channels has a respective inner diameter between 0.75 millimeter and 0.25 millimeter.
Type: Application
Filed: Jun 8, 2011
Publication Date: Dec 13, 2012
Inventor: Gary D. Conley (Saratoga, CA)
Application Number: 13/155,602
International Classification: F24J 2/05 (20060101); B21D 53/06 (20060101); F24J 2/24 (20060101); F24J 2/12 (20060101); F24J 2/48 (20060101);