GB2027873A - Solar collector - Google Patents

Abstract

A solar collector comprises a housing 10, inside which is arranged an absorber in the form of a pipe system 18 through which a heat exchange medium flows, the housing being provided with a cover 14 through which radiation can pass. The pipe system 18 includes at least one pipe 19 of substantially circular cross-section, the pipes being arranged in spaced relationship in one plane. A reflector 17 is disposed below the pipes 19. <IMAGE>

Description

SPECIFICATION Solar collector The invention relates to a solar collector comprising a housing, in the interior of which is arranged an absorber in the form of a pipe system through which a heat exchange medium can be passed in order to be heated by incident solar radiation.

Known solar collectors or solar panels of this kind consist of a substantially closed-off housing which is provided with a glass cover.

The glass cover permits short-wave high-energy solar radiation to penetrate into the housing without very great losses, but it forms a barrier against the long-wave thermal radiation occurring in the collector. In this way, the pipe system within the housing is heated up by thermal absorption. The principle on which solar collectors operate is that when solar radiation is absorbed in a black body, the latter warms up and in turn emits radiation in the form of heat. The covering used takes the form of special solar glasses which, over the entire wave-length range of solar radiation, have a penetrability of more than 90%, and for long-wave thermal radiation, a penetrability of less than 2%. Thus, given good thermal insulation against exterior influences, a noload temperature of 1 80 C occurs in the collector.

In the known solar collectors, the absorber consists of a pipe system of flat cross-section.

These flat pipes have a planar upper face presented to the covering, and a planar lower face presented to a reflector. The highest values for the absorption of energy from solar radiation are obtained when the radiation is incident vertically on the absorbing surface.

With the solar collector disposed in the optimum manner, this requirement is met only once daily, at mid-day when the sun is in the south. The sharper the angle at which the rays of the sun strike the absorbing surface, the less will be the amount of radiation absorbed and the greater will be the amount of radiation reflected. This means that with a sharp angle of incidence, a large part of the solar radiation is reflected out of the collector before having been converted into heat. With a flat absorption element, the effective surface is roughly only as great as that (planar) pipe face presented to the cover.

The object of the present invention is to provide a solar collector wherein, for different directions of incidence of the rays, the absorption capacity is increased above that of the known collectors, and which converts a large quantity of short-wave radiation into heat, particularly in the case of diffuse radiation (when the sky is cloudy).

According to the invention, a solar collector comprises a housing including a cover adapted to permit solar radiation to pass therethrough into the interior of the housing in which is arranged an absorber in the form of a pipe system through which a heat exchange medium can be passed, wherein the pipe system comprises at least one pipe of substantially circular cross-section.

In the case of a round pipe and over a longer period of the day during travel of the sun, a more favourable angle of impingement of solar radiation occurs than in the case of a flat pipe. Although the flat pipe, when in the ideal position in which it is aligned with the sun, provides higher absorption values than does a pipe having a circular cross-section, a circular pipe has been found to be more favourable in the case of diffuse radiation or radiation impinging on the solar collector at an oblique angle, since a portion of the radiation is always incident upon a portion of the pipe surface in the perpendicular or almost perpendicular direction. Therefore, compared with the flat pipe, the absorption capacity of a round pipe is very much less dependent upon the angle of incidence of the radiation and enables an energy yield to be obtained that is more uniform in terms of time.

To further improve the absorption of the solar radiation the pipes of the pipe system may be arranged in spaced relationship in a common plane, Preferably also a reflector is disposed below the pipe system. The reflector directs the radiation passing between the pipes to the lower faces of the pipes where it impinges practically perpendicularly on the pipe surface and is therefore absorbed to a large extent.

The reflector may consist of a planar reflecting layer, which may rest on the inner surface of a rear wall of the housing or may be arranged at a distance above this inner surface. The pipe system may be disposed at a distance above the reflecting layer or may be connected to it physically. Physical connection of the pipe system to the reflector results in improved dissipation of heat from the reflector.

Alternatively, the reflector may be substantially zig-zag in cross-section, and each length of the pipe forming the pipe system may be secured to and extends parallel with a crest of the reflector. The surface of the reflector is increased by the zig-zag shape. The sharp angles of the reflector permit the system to be more fully exploited and increases the transmission of heat to the pipe system even when sunlight is received at an inclined angle.

The zig-zag reflector can be sharply angled or rounded or flattened on at least one of its two faces.

The pipe system is preferably made of copper, the good thermal conductivity of which is well known. The pipe system may consist, for example, of a coil, one end of which is connected to a pipe for supplying the heat exchange medium, while its other end is con nected to a delivery pipe.

Some embodiments of the invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross-section through a first embodiment of solar collector having a planar reflector layer; Figure 2 is a diagram illustrating the effect of obliquely incident sunrays and the effect of scattered radiation; Figure 3 is a diagrammatic cross-section through a second embodiment of solar collector having a reflector of zig-zag shape, and Figure 4 is a diagrammatic cross-section through a further embodiment having a planar reflector layer.

The solar collector illustrated in cross-section in Fig. 1 consists of a housing 10, the frame 11 of which is made of weather-resisting timber treated with a jet-black matt impregnating substance. The rear side of the housing is formed by a water-proof chipboard 12, which carries a polyurethane foam insulating layer which resists high temperature. The entire housing 10 is enclosed in a sectional structure 1 3 made of anodized aluminium profiles.

The top of the housing 10, formed as a shallow box, is closed off by a plate 14 of special solar glass. The metal profiled structure 13, which encloses the frame 11 of the housing, overlaps the edges of the glass plate 14, so that the latter is firmly connected to the frame of the housing. The lower face of the glass plate 14 rests, by way of sealing strips 15, on a projection 1 6 extending around the interior of the housing 10.

The bottom plate 14 is covered by a planar reflector layer 1 7 made of aluminium. Disposed above the reflector layer 1 7 is the pipe system 1 8 which consists for example, of a coil. In the present case, the meandering pipe 1 9 has a circular cross-section, is made of copper and has a thin wall. The ends of the pipe are connected to ports (not illustrated), which lead out of the housing 1 0. The heating medium may be a mixture of water and anti-freeze agent which passes through the pipe.

The conditions that affect the absorber 1 9 as regards reflection and absorption are illustrated diagrammatically in Fig. 2. When the sun is shining (as indicated on the left in Fig.

2), part of the solar radiation is always incident perpendicularly on the pipe walls irrespective of the angle of incidence of the rays of the sun. Thus, in sunshine, the energy yield of this collector is largely independent of the direction of incidence.

Diffuse radiation (shown on the right in Fig.

2) is incident on the pipe 1 9 in various directions, i.e. between the pipe sections.

Some of the radiation is reflected by the wall of the pipes and then at some time strikes another pipe, and part is reflected at the reflector 1 7 and from here passes to the rear side of the pipes. On the whole, the absorber having a circular or at least a substantially circular cross-section offers a much larger absorption area for diffuse radiation than does a fiat absorber. This is of great importance particularly for providing maximum uniformity in energy yield when the system is used in zones of moderate climate in which cloud cover frequently occurs.

Referring to Fig. 3, in this embodiment the housing 10 is designed in substantially the same way as in the embodiment of Fig. 1.

The only difference is that in the Fig. 3 embodiment the projection 1 6 of Fig. 1 is replaced by a peripheral shoulder 21 on the frame 11 for supporting the edges of the glass plate 14. A sealing strip 1 5 is fitted between the edge of the glass plate 1 4 and the shoulder 21.

Within the housing, the pipe system 18, which consists for example of a coil, is firmly connected to a copper reflector 20. The crosssection of the reflector 20 is of substantially zig-zag form, and each run 1 9 of the pipe system 1 8 is secured to, e.g. soldered, and extends parallel to a crest of the reflector 20.

In the Fig. 3 embodiment, the lower crests 23 of the reflector 20 have sharp angles, whereas the upper crests are flattened, or are preferably slightly concave so as to receive the pipes 19. The lower crests 23 are secured to the inner surface of the rear wall 1 2 of the housing. The angled portions of the reflector 20 are such that an unoccupied or open gap is formed between each two adjacent parallel pipe runs 19, so that the rays of the sun strike the inclined faces of the reflector and these faces transmit the heat to the pipes 19.

A further embodiment of the collector in accordance with the invention is shown in Fig.

4. In this case a planar reflector layer 22 made of copper is arranged at a distance above the inner surface of the rear wall 1 2 of the housing, and the pipes 1 9 of the system 1 8 are likewise physically connected, e.g. by soldering, to the surface of this reflector 22.

The outer edges 24 of the reflector 22 that are parallel with the pipe runs 1 9 are upwardly curved to match the radius of curvature of the pipe runs 1 9. Solar radiation strikes the reflector 22 by way of the gaps between the pipe runs, and the absorbed heat is reflected towards the pipe system 1 8.

Claims (14)

1. A solar collector which comprises a housing including a cover adapted to permit solar radiation to pass therethrough into the interior of the housing, in which is arranged an absorber in the form of a pipe system through which a heat exchange medium can be passed, wherein the pipe system comprises at least one pipe of substantially circular cross-section.

2. A solar collector according to Claim 1, wherein the pipes of the pipe system are arranged in spaced relationship in a common plane.

3. A solar collector according to Claim 1 or Claim 2, further comprising a reflector disposed below the pipe system.

4. A solar collector according to Claim 3, wherein the reflector is a planar reflecting layer.

5. A solar collector according to Claim 4, wherein the planar reflecting layer rests on the inner surface of a rear wall of the housing.

6. A solar collector according to Claim 4, wherein the planar reflecting layer is arranged at a distance above the inner surface of a rear wall of the housing.

7. A solar collector according to any one of Claims 4 to 6, wherein the pipe system is arranged at a distance above the reflecting layer.

8. A solar collector according to any one of Claims 4 to 6, wherein the pipe system is physically connected to the reflecting layer.

9. A solar collector according to Claim 3, wherein the reflector is of substantially zig-zag cross-section and each length of pipe forming the pipe system is secured to and extends parallel with a crest of the reflector.

1 0. A solar collector according to Claim 9, wherein the reflector is of acute-angled shape in at least one of its two planes.

11. A solar collector according to Claim 9, wherein the reflector is of rounded shape at least in one of its two planes.

1 2. A solar collector according to Claim 9, wherein the reflector is of flattened form in at least one of its two planes.

1 3. A solar collector according to any one of Claims 1 to 12, wherein the pipe system is made of copper.

14. A solar collector substantially as here it before described with reference to the accompanying drawings.