GB2213257A

GB2213257A - Solar collectors

Info

Publication number: GB2213257A
Application number: GB8828188A
Authority: GB
Inventors: Hossein Valizadeh
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-12-03
Filing date: 1988-12-02
Publication date: 1989-08-09
Also published as: ZA889064B; IE873291L; GB8828188D0

Abstract

A solar collector comprises an evacuated envelope 10 transmissive of solar radiation, an absorber plate 11 within the envelope for absorbing solar radiation passing through the envelope, and a heat pipe 12 in thermal contact with one face 14 of the plate internally of the envelope and which extends out of the envelope to a condenser. The absorber plate 11 is coated on each face 13, 14 with a layer 15 of a material which increases the heat absorbance efficiency of the plate. The collector further comprises a reflector means 17 within the envelope located on that side of the absorber plate whose face is in thermal contact with the heat pipe 12, the reflector means acting both to reflect back to the said face 14 any heat radiated by the said face and to reflect solar radiation falling upon the reflector onto the said face. The reflector means may be a u-shaped construction or may be a coating on the envelope wall. <IMAGE>

Description

DESCRIPTION OF INVENTION Title: "Reducing heat loss in evacuated solar collectors This invention relates to a solar collector of the kind comprising an evacuated envelope transmissive of solar radiation, an absorber plate within the envelope for absorbing solar radiation passing through the envelope, and a heat pipe in thermal contact with the plate internally of the envelope and which extends out of the envelope to a condenser, the heat pipe containing a working fluid which is evaporated in the envelope by heat from the plate and condensed externally of the envelope in the condenser.

Solar collectors of the aforementioned kind are described in UK Patent Specifications GB 2 103 350, GB 2 117 104 and GB 2 131 155, whose contents are hereby incorporated by reference.

Figure 1 is an enlarged cross-section through a typical prior art solar collector, for example that shown in GB 2 103 350. Essentially, the solar collector comprises an evacuated envelope in the form of an elongated glass tube 10, and a solar radiation absorber plate in the form of an elongated fin 11 arranged generally axially along the tube 10. A metal heat pipe 12 is in thermal contact with one side of the fin 11, and the pipe 12 extends axially along the fin to emerge from the tube 10 via a hermetic seal, not shown, for connection to a condenser, also not shown. As mentioned, this pipe contains a working fluid which is evaporated in the tube 10 by heat absorbed by the fin 11 and condensed externally of the tube 10 in the condensor.

Since the fin 11 is typically of aluminium, it is coated with a layer of material which increases its heat absorbance efficiency, such as by painting with matt black paint. All manufacturers of evacuated solar collectors place much emphasis on the absorption of radiant solar energy not only by the front face 13 of the fin, but also (in direct or reflected form) by the back face 14 which contacts the heat pipe 12.

Accordingly, Figure 1 shows that a coating 15 of e.g.

black paint is applied to both surfaces of the fin 11, not only the front face 13 which in use faces the sun 16, but also its back face 14.

However, being a good heat absorber, the layer 15 on the back face 14 is also a good heat emitter, and therefore serves as an undesireable source of heat loss from the fin 11. Indeed, the higher the temperature of the fin, the greater the heat loss, since the coefficient of heat loss of a black surface is proportional to T, where T is the temperature.

It is an object of the invention to provide a construction of solar heat collector where this heat loss is reduced.

According to the invention, there is provided a solar collector comprising an evacuated envelope transmissive of solar radiation, an absorber plate within the envelope for absorbing solar radiation passing through the envelope, and a heat pipe in thermal contact with one face of the plate internally of the envelope and which extends out of the envelope to a condenser, wherein the said absorber plate is coated on each face with a layer of a material which increases the heat absorbance efficiency of the plate, the collector further comprising a reflector means within the envelope located on that side of the absorber plate whose face is in thermal contact with the heat pipe, the reflector means acting both to reflect back to the said face any heat radiated by the said face and to reflect solar radiation falling upon the reflector onto the said face.

In this case the heat emitted by the back face is reflected back onto the face, and in addition at least some of the solar radiation falling on the reflector will also be reflected onto the back face. Hence, the overall efficiency is increased.

The invention will further be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1, previously described, is a cross-section through a known construction of solar collector; Figure 2 is a similar cross-section through an embodiment of a solar collector according to the invention; and Figure 3 is a similar cross-section through another embodiment of a solar collector according to the invention.

In the drawings, similar components have been given similar reference numerals throughout.

In Figure 2, the fin 11 is provided on both front and back surfaces 13 and 14 respectively with a coating of heat absorbing material 15, as in Figure 1, but in addition a reflector member 17 is provided inside the tube 10. The reflector member 17, which is generally 'U' shaped, extends along the length of the fin 11 on the side facing the back face 14 of the fin, and is symetrically disposed relative to the back face 14 and heat pipe 12. The reflector member 17 is slightly wider than the fin 11, so that in addition to reflecting back any radiation 18 emitted by the back face 14 of the fin 11, it can also reflect onto the back face 14 at least some of the radiation 19 falling directly on the reflector 17 from the sun 16.

As in the preceding construction, the solar collector is highly elongated in the direction out of the plane of the figure as described and shown in the aforementioned Patent Specifications.

Apart from the differences noted above, the solar collector may be of conventional construction as described in the aforementioned Patent Specifications.

In Figure 3 there is shown another embodiment of the invention in which a thin layer of reflective silver coating film 17' is provided on a portion of the inner surface of the tube 10, facing the back face 14 of the fin. This reflective coating 17' functions the same as the reflector 17 of Figure 2. The reflective coating 17' extends along the length of the fin 11 on the side facing the back face 14 of the fin.

Claims

CLAIMS:

1. A solar collector comprising an evacuated envelope transmissive of solar radiation, an absorber plate within the envelope for absorbing solar radiation passing through the envelope, and a heat pipe in thermal contact with one face of the plate internally of the envelope and which extends out of the envelope to a condenser, wherein the said absorber plate is coated on each face with a layer of a material which increases the heat absorbance efficiency of the plate, the collector further comprising a reflector means within the envelope located on that side of the absorber plate whose face is in thermal contact with the heat pipe, the reflector means acting both to reflect back to the said face any heat radiated by the said face and to reflect solar radiation falling upon the reflector onto the said face.

2. A solar collector as claimed in Claim 1, wherein the reflector means comprises a reflector member mounted within the envelope.

3. A solar collector as claimed in Claim 2, wherein the reflector member is substantially 'U' shaped.

4. A solar collector as claimed in Claim 1, wherein the reflector means comprises a reflective coating provided on the inner surface of the envelope.

5. A solar collector as claimed in Claim 1, substantially as hereinbefore described with reference to and as illustrated in Figure 2 or Figure 3.

6. Any novel feature or combination of features described herein.