WO2010032110A1

WO2010032110A1 - Evacuated tube solar collector device

Info

Publication number: WO2010032110A1
Application number: PCT/IB2009/006862
Authority: WO
Inventors: Geoffrey Harding
Original assignee: Kloben S.A.S. Di Turco Adelino E C.
Priority date: 2008-09-18
Filing date: 2009-09-17
Publication date: 2010-03-25

Abstract

Evacuated tube solar collector device (1) comprising an envelope tube (2), an absorber tube (3) placed inside the envelope tube (2) and adapted to receive a heat transfer fluid and a coating layer (4) applied on the outer surface (3 c) of the absorber tube (3) for reducing its emissivity, the outer surface (3c) of the absorber tube (3) comprising at least an uncoated zone (5) that is free from said coating layer (4).

Description

EVACUATED TUBE SOLAR COLLECTOR DEVICE

Technical Field

The present invention relates to an evacuated tube solar collector device and to a method for limiting the stagnation temperature of an evacuated tube solar collector device. Background Art

An evacuated tube solar collector device is typically composed of two concentric glass tubes: an envelope tube closed off by a dome at one end and an absorber tube of smaller diameter than the envelope, also closed off by a dome at one end, which is placed inside the envelope tube and sealed to the envelope by a glass seal at the open ends of the tubes. The domed end of the absorber tube is supported in a concentric position within the envelope by means of a metal spring clip, the design of which is well known to those skilled in the art of tube collector design. Inside the absorber tube is circulated a heat transfer fluid. The volume between the two glass tubes is evacuated in order to minimize the conduction and convention heat losses from the absorber tube. The outer surface of the absorber tube is completely coated with a coating layer of a selective surface material with high absorption of solar radiation and low emission of thermal radiation, in order to maximize the absorption of solar energy and at the same time minimizing the heat loss by radiation of infra-red when the absorber tube is heated by the sunlight.

The energy radiated by a surface at a temperature T is characterized by a temperature dependent parameter called the emissivity "e", which is the fraction of the maximum possible thermal energy which can be radiated by a surface at temperature T. The energy radiated per unit area of the surface is proportional to e^χT⁴. A typical good quality selective surface has e<0.10, which means that less than 10% of the maximum possible thermal energy is radiated from the selective surface. The combined properties of vacuum insulation, high absorption for solar radiation and low emission for radiation of heat results potentially in high heat extraction efficiency for the evacuated collector because little of the absorbed heat energy is lost by conduction or radiation from the absorber tube. Useful heat energy is typically extracted from the absorber tube by circulating a heat transfer fluid such as water into and out of the glass absorber tube via the open end.

These evacuated tube solar collector devices however do have some drawbacks. Indeed, if the heat extraction from the absorber tube is interrupted, for example due to removal of the circulating heat transfer fluid from the glass absorber tube itself, or cessation of flow of the heat transfer fluid, the temperature of the absorber tube and its contents rises to a maximum temperature, referred to as the stagnation temperature, at which the solar radiation absorbed is equal to the energy radiated by the coated outer surface of the absorber tube. At the stagnation temperature the heat extraction efficiency of the device is effectively zero.

The problem is that under stagnation conditions the extremely high temperature reached may result in damage to some of the components to which the absorber tube is attached, damage to fluid remaining within the absorber tube, and to the glass collector itself if cold fluid is suddenly reintroduced into the absorber tube. A proposed method for limiting the stagnation temperature of a collector tube is disclosed in US patent 4,834,066 which describes the incorporation of a low pressure gas in the enclosed volume between the two tubes with properties such that it adsorbs onto the selective surface coating layer at low temperature. Thus at low temperatures the pressure of gas in the enclosed volume is very low and heat conduction losses from the absorber tube are very low. At higher temperatures, the gas desorbs from the coating layer resulting in relatively high gas pressure in the enclosed volume and considerably higher heat losses occur from the absorber tube due to thermal conduction through the gas, with consequent limitation of the stagnation temperature. This proposed method is very difficult to realize in practice due to the required properties of both gas and selective surface coating layer. A reliable method of limiting the stagnation temperature of a collector tube involves increasing the emissivity of the selective surface coating layer. The design of the coating layer and the materials used in fabrication of the coating layer can be chosen to produce various higher values of emissivity, and consequent lower stagnation temperature, however this approach will generally require significant changes in a production process designed to produce one particular type of coating layer. Disclosure of the Invention The aim of the present invention is to provide an evacuated tube solar collector device that permits to reduce the stagnation temperature to various chosen values without significant alteration of the device structure or of the materials used for the coating layer.

Within this aim, an object of the present invention is to provide an evacuated tube solar collector device that permits to reduce the stagnation temperature without using any incorporated gas or particular selective surface coating. Another object of the present invention is to provide a method for producing a coated absorber tube with a reduced stagnation temperature. Within the scope of such technical aim, another object of the present invention is to cater for the above aims with a simple structure, of relatively practical implementation, safe to use and with effective operation, as well as having a relatively low cost.

This aim and these and other objects which will become better apparent hereinafter are achieved by the present evacuated tube solar collector device, comprising: an envelope tube; an absorber tube placed inside the envelope tube and adapted to receive a heat transfer fluid; a coating layer applied on the outer surface of the absorber tube to produce a selective surface; characterized in that the outer surface of the absorber tube comprises at least an uncoated zone that is free from said coating layer.

The aim and the objects of the present invention are also achieved by the present method for limiting the stagnation temperature of an evacuated tube solar collector device, characterized in that it comprises the following steps: furnishing an absorber tube adapted to be placed inside an envelope tube of an evacuated tube solar collector device and adapted to receive a heat transfer fluid; retaining at least a protective element on a relative zone of the outer surface of the absorber tube; applying a coating layer on the outer surface of the absorber tube; - removing said protective element, the zone on which was retained said protective element defining an uncoated zone. Brief Description of the Drawings

Further characteristics and advantages of the present invention will appear even more evident from the detailed description of a preferred, but non-exclusive, embodiment of an evacuated tube solar collector device, illustrated indicatively by way of non limiting example, in the attached drawings wherein: the figure 1 is a side view of an evacuated tube solar collector device according to the present invention; the figure 2 is a cross sectional view of the device of figure 1; the figure 3 is a cross sectional view of an evacuated tube solar collector device according to the present invention with containing means of a heat transfer fluid; the figure 4 illustrates the phase of retaining a protective element on the outer surface of the absorber tube in a first form of embodiment of the method according to the present invention; the figure 5 is a cross sectional view of the absorber tube of figure 4; the figure 6 illustrates the phase of retaining a protective element on the outer surface of the absorber tube in a second form of embodiment of the method according to the present invention. Ways of .carrying out the Invention

With particular reference to such figures, globally indicated by 1 is an evacuated tube solar collector device.

The device 1 comprises an envelope tube 2 and an absorber tube 3 placed inside and sealed to the envelope tube 2 and adapted to receive a heat transfer fluid. The absorber tube 3 has a closed end 3a and an open end 3b opposite to the closed end 3 a, while the envelope tube 2 has a closed end 2a and is sealed to the absorber tube 3 in correspondence of its open end 3b. More particularly, as shown in figure 1, the closed end 3a is dome-shaped and is supported by a spring clip 20 interposed between the absorber tube 3 and the envelope tube 2. Preferably, both the envelope tube 2 and the absorber tube 3 are made of glass and are cylindrical. However absorber tubes 3 and/or envelope tubes 2 with alternative cross sectional shape are not excluded by the present invention. The device 1 comprises a coating layer 4 applied on the outer surface 3 c of the absorber tube 3 to produce a selective surface.

More particularly, the coating layer 4, that defines a coating surface 4a, is made of a material with a high absorption of solar radiation and low emission of thermal radiation. A typical value of the coating layer emissivity is about 0,08 which greatly reduces the heat loss by radiation.

According to the present invention, the outer surface 3 c of the absorber tube 3 comprises at least an uncoated zone 5 free from the coating layer 4. The uncoated zone 5 consists of a non-coated glass surface which typically exhibits a emissivity value of about 0,85. Consequently by adjusting the total area of the uncoated zone 5, the average emissivity of the absorber tube 3, calculated considering the area of the coating surface 4a and of the uncoated zone 5, may be varied over a wide range of values, which allows the stagnation temperature of the device 1 to be varied over a wide range. For example, if the coating surface 4a has emissivity of about 0.08, and the area of the uncoated zone 5 corresponds to about 20% of the outer surface 3 c of the absorber tube 3, the average emissivity of the absorber tube 3 will be increased to about 0.23 (0.8x0.08 + 0.2x0.85) and the stagnation temperature of the absorber tube itself will be reduced typically from about 250⁰C to less than 140⁰C.

The higher average emissivity results in higher radiation heat loss from the device 1 during normal operation at low temperature; however due to the T⁴ dependence of radiation heat loss, the heat loss at low operating temperature will be relatively low. Furthermore, although the uncoated zone 5 is not coated with a solar energy absorbing surface, the uncoated zone 5 absorbs sunlight quite effectively by allowing about 90% of incident sunlight to pass through the non-coated glass to the interior of the absorber tube where it will be substantially absorbed.

Advantageously, the uncoated zone 5 is disposed downwards in the functioning of the device 1 itself, i.e. on the part of the absorber tube 3 opposite to its part directly faced to the sun, because generally very little sunlight is incident on the underside of the absorber tube 3.

The uncoated zone 5 may extend itself at least along part of the entire length of the absorber tube 3.

Preferably, as shown in figure 1, the uncoated zone 5 extends itself along the entire length of the absorber tube 3, in which case the stagnation temperature is reduced to approximately the same value along the entire length of the tube. Alternatively, in some collector designs the uncoated zone 5 may advantageously extend for only a proportion of the length of the absorber tube 3, for example 10% to 40% of the total length closest to the open end 3b. In such a case the stagnation temperature is reduced along this portion of the absorber tube 3 closest to its open end 3b while the portion of absorber tube 3 further from the open end itself has higher stagnation temperature. Such a shortened length of the uncoated zone 5 has the advantage of reducing the radiation heat loss from the major part of the absorber tube 3 during heat extraction, while reducing the stagnation temperature close to the open end 3b of the absorber tube 3 where other collector components, for example pipes made from low melting point polymer, may be attached. More particularly, the outer surface 3 c of the absorber tube 3 may contain one or more uncoated zones 5 strip-shaped of approximately uniform width, and extending along at least a part of the length, preferably along the entire length, of the absorber tube 3, substantially parallel to its axis.

Obviously, not excluded by the present invention are other forms of embodiment, not represented in the alleged figures, with different shapes of the uncoated zones 5 that, for example, may consist in a regular array of rings circumscribing the absorber tube 3, or for example a triangular shape with greatest width near the open end 3b varying to very small width at some distance from the open end itself. Preferably, the total area of the uncoated zones 5 corresponds to 2% - 40% of the total outer surface area of the absorber tube 3.

As above described, the absorber tube 3 is adapted to receive a heat transfer fluid.

More particularly, the heat transfer fluid may flow directly inside the absorber tube 3 or the device 1 may comprise containing means 6 for the heat transfer fluid, disposed inside the absorber tube itself.

In the form of embodiment of figure 3, the containing means 6 comprises at least a U-shaped metal tube 7 adapted to circulate the heat transfer fluid and a metal layer 8 contacting the outer surface of the metal tube 7. More particularly, the outer surface of the metal tube 7 is in intimate thermal contact with a suitably curved high thermal conductivity metal layer 8 adapted to maintain good thermal contact with at least a major part of the inner surface 3d of the absorber tube 3. Advantageously, the inner surface 3d comprises at least an uncovered area 9 that is not in contact with the metal layer 8.

Preferably, the metal layer 8 is designed so that it does not extend completely in correspondence with the part of the absorber tube 3 on which is defined the uncoated zone 5, i.e. the lowest part of the absorber tube 3 in figure 3. In other words, the uncovered area 9 is substantially aligned to the uncoated zone 5. More particularly, in the case that the absorber tube 3 is substantially cylindrical, the radial lines that connect the extremities of the metal layer 8 to the axis of the absorber tube 3 are angularly spaced at a larger angle from the radial lines that connect the extremities of the coating layer 4 with the same axis. In other words, the uncovered area 9 of the inner surface of the absorber tube 3 which has no contact with the metal layer 8 is larger in extent than the area of the uncoated zone 5 on the outer surface of the absorber tube itself. Thus, during normal operation when heat is being extracted by circulation of heat transfer fluid through the metal tube 7, the high thermal conductivity metal layer 8 does not conduct heat directly to the uncoated zone 5 and the very low thermal conductivity of glass results in minimal energy conducted around the absorber tube 3 to the uncoated zone 5. So, the uncoated zone 5 will remain at a lower temperature than the coated portion of the outer surface 3 c and consequently radiate less energy. When the absorber tube 3 reaches the stagnation temperature, the higher temperature of the absorber tube 3 results in considerably more heat energy being transferred to the uncoated zone 5 due to the thermal radiation inside the absorber tube 3, the conduction and convection in the air inside the tube and some thermal conduction around the glass. So, the temperature of the uncoated zone 5 increases, the uncoated zone then itself radiates energy strongly and the stagnation temperature is limited. In another form of embodiment, not represented in the alleged figures, the fluid containing means 6 comprises a heat pipe. In this form of embodiment, the outer surface of the evaporator of the heat pipe is in intimate thermal contact with a suitably curved high thermal conductivity metal layer adapted to maintain good thermal contact with at least the major part of the inner surface 3d of the absorber tube 3. So, the structure of the device 1 in this form of embodiment is very similar to the one with the U- shaped metal tube 7, in which the two arms of the metal tube 7 are replaced by the single evaporator of the heat pipe. As is well known, the heat pipe comprises a condenser and the heat transfer fluid circulating inside the heat pipe evaporates due to the heat of the absorber tube 3 and reaches the condenser where it transfers its heat by conduction to an external fluid. The present invention also relates to a method for limiting the stagnation temperature of an evacuated tube solar collector device, i.e. a method for producing a device like that as above described.

The method according to the present invention comprises, first of all, the step of furnishing an absorber tube 3 adapted to be placed inside the envelope tube 2 of an evacuated tube solar collector device 1 and adapted to receive a heat transfer fluid. Preferably, the absorber tube 3 is made of glass and is provided with a closed end 3a and with an open end 3b.

Subsequently, the method according to the present invention comprises the step of retaining at least a protective element 10 on a relative zone of the outer surface 3c of the absorber tube 3.

This step of retaining the protective element 10 may be achieved in different ways, as represented in the figures 4 and 6. More particularly, in first form of embodiment, it is possible to retain at least a protective element 10, at least partially made with a ferromagnetic material such as steel, on the outer surface 3 c by applying one or more magnetic elements 11 on the inner surface 3d of the absorber tube 3. So, the protective element 10 is maintained in position against the outer surface 3 c of the absorber tube 3 by the magnetic attraction exerted by the magnetic elements 11. Preferably, as shown in figure 4, the magnetic elements 11 are fixed to a non-ferromagnetic strip 12 disposed between the magnetic elements themselves and the inner surface 3d of the absorber tube 3. Advantageously, as shown in figure 5, the strip 12 has a curvature which matches the curvature of the inner surface 3d and the protective element 10 has a radius of curvature which matches, or is less than, the radius of curvature of the outer surface 3c. Alternatively the protective element 10 may be easily formed from a metal strip bent by a small angle with bending axis along the centre of the strip. In a second form of embodiment, represented in figure 6, the retaining of the protective element 10 is achieved by clamping the protective element itself against the surface of the absorber tube using one or more fine wires 13, for example made of metal, which pass around the outer surface 3 c of the absorber tube 3 not covered by the protective element 10 and are attached to the protective element itself, or which pass around the outer surface of the protective element 10.

Advantageously, each wire 13 comprises a relative tensioning spring 14 to facilitate the clamping of the protective element 10 in close contact with the absorber tube 3. In a further form of embodiment, the protective element 10 may be constituted by an adhesive tape applied to the outer surface 3c. Generally, this form of embodiment is not adapted to be used with elevated temperature, because most adhesive tapes can only withstand temperatures of < 80⁰C. After having retained the protective element 10 on the outer surface 3 c, the method according to the invention comprises the step of applying the coating layer 4 on the outer surface 3 c of the absorber tube 3. As known by the skilled person, the preferred method for applying the selective surface coating layer 4 onto the glass absorber tube 3 is by the sputtering of metal electrodes in a high vacuum chamber. In the sputtering process, individual atoms are ejected from the surface of a metal electrode by bombardment with argon ions. The ejected metal atoms will coat the nearby substrate with a sputtered film of the metal, or of metal plus non-metal atoms derived from a reactive gas such as nitrogen introduced to the vacuum chamber. The coating layer 4 can also be produced in a vacuum chamber by evaporation, in which a metal or metal compound vapor is produced by heating a metallic or metal compound material to extremely high temperature until it evaporates. The nearby substrate will be coated by the vapor produced.

After application of the coating layer 4 with one of the methods above described, it is necessary to removing the protective element 10. The zone of the absorber tube 3 on which the protective element 10 was retained defines the uncoated zone 5 of the absorber tube 3. It is clear that the total area of the uncoated zone or zones 5 required to limit the stagnation temperature to a specific value may be calculated approximately given the solar energy absorbed by the device 1, the emissivity value for the particular coating layer 4 and the emissivity value of about 0.85 for the uncoated zone 5. Alternatively, absorber tubes 3 incorporating uncoated zones 5 of various widths or areas may be fabricated relatively easily and the stagnation temperature of the devices 1 incorporating these absorber tubes measured in strong sunlight. Thus the ideal width or area of the uncoated zone 5 may be determined by experiment. The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims.

All the details may further be replaced with other technically equivalent ones. In practice, the material used, as well as the shapes and the dimensions, may be any according to requirements without thereby abandoning the scope of the appended claims.

Claims

1) Evacuated tube solar collector device (1), comprising: an envelope tube (2); an absorber tube (3) placed inside the envelope tube (2) and adapted to receive a heat transfer fluid; a coating layer (4) applied on the outer surface (3c) of the absorber tube (3) to produce a selective surface; characterized in that the outer surface (3 c) of the absorber tube (3) comprises at least an uncoated zone (5) that is free from said coating layer (4). 2) A device (1) according to claim 1, characterized in that said uncoated zone (5) is disposed downwards in the functioning of the device (1) itself. 3) A device (1) according claim 1 or 2, characterized in that said uncoated zone (5) substantially extends itself along at least a part of the entire length of the absorber tube (3). 4) A device (1) according to claim 3, characterized in that said uncoated zone (5) extends itself along the entire length of the absorber tube. 5) A device (1) according to one or more of the preceding claims, characterized in that it comprises at least an open end and characterized in that the uncoated zone (5) is located adjacent to said open end. 6) A device (1) according to one or more of the preceding claims, characterized in that said uncoated zone (5) is strip-shaped.

7) A device (1) according to one or more of the preceding claims, characterized in that the total area of said at least an uncoated zone (5) corresponds to 2% - 40% of total outer surface (3 c) area of the absorber tube (3).

8) A device (1) according to one or more of the preceding claims, characterized in that it comprises containing means (6), disposed inside the absorber tube (3), for the heat transfer fluid.

9) A device (1) according to claim 8, characterized in that said containing means (6) comprises at least an U-shaped metal tube (7) adapted to circulate the heat transfer fluid and comprises a metal layer (8) contacting the outer surface of the metal tube (7) and at least a part of the inner surface (3d) of the absorber tube (3), said metal layer (8) being suitable to maintain thermal contact between the metal tube (7) and the absorber tube (3).

10) A device (1) according to claim 9, characterized in that the inner surface (3d) of the absorber tube (3) comprises at least an uncovered area (9) that is free from contact with the metal layer (8).

11) A device (1) according to claim 10, characterized in that said uncovered area (9) is disposed substantially in correspondence of the uncoated zone (5).

12) A device (1) according to claim 11, characterized in that the absorber tube (3) is cylindrical and the angular width of said uncovered area (9) is larger than the angular width of the uncoated zone (5).

13) Method for limiting the stagnation temperature of an evacuated tube solar collector device, characterized in that it comprises the following steps: furnishing an absorber tube (3) adapted to be placed inside an envelope tube (2) of an evacuated tube solar collector device (1) and adapted to receive a heat transfer fluid; retaining at least a protective element (10) on a relative zone of the outer surface (3 c) of the absorber tube (3); applying a coating layer (4) on the outer surface (3 c) of the absorber tube

(3); - removing said protective element (10), the zone on which was retained said protective element (10) defining an uncoated zone (5).

14) Method according to claim 13, characterized in that said retaining of a protective element (10) is achieved by applying one or more magnetic elements (11) on the inner surface (3d) of the absorber tube (3), the protective element (10) being at least partially made from a ferromagnetic material.

15) Method according to claim 13, characterized in that said retaining of a protective element (10) is achieved by clamping the protective element (10) against the surface of the absorber tube (3) with one or more fine wires (13) associated to the protective element itself and which pass around the outer surface (3c) of the absorber tube (3) not covered by the protective element itself(10).

16) Method according to claim 15, characterized in that said fine wires (13) incorporate a relative tensioning spring (14).