WO2007026311A1

WO2007026311A1 - Solar energy collector

Info

Publication number: WO2007026311A1
Application number: PCT/IB2006/053013
Authority: WO
Inventors: Christian Lippens
Original assignee: Vanderstraeten, Luc
Priority date: 2005-08-30
Filing date: 2006-08-30
Publication date: 2007-03-08
Also published as: BE1016740A5

Abstract

This invention relates to a solar collector (1), comprising a tubular housing (2) which is provided inside the space of the housing (2) with an absorber (3) for absorbing solar radiation (4) across virtually the entire surface of the tubular housing (2); one or more surfaces (5) reflecting solar radiation for indirectly irradiating the part of said absorber (3) which is located on the shadow side of the tubular housing (2), at least one of the reflecting surfaces (5) extending up to at least the horizontal tangent (A) on the outermost point of the absorber (3) which is located on the side of the housing (2) on which direct solar radiation impinges; in which the solar collector (1) according to the invention is characterized in that the surfaces (5) reflecting the solar radiation and the absorber (3) are designed such that all solar radiation (4) incident at right angles impinges on the absorber (3), on the one hand directly and on the other hand by reflection on the reflecting surfaces, in such a manner that the sine of the angle of incidence of said radiation on the absorber (3) is substantially equal to 1.

Description

SOLAR ENERGY COLLECTOR

This invention firstly relates to a solar collector, comprising: a tubular housing which is provided inside the space of the housing with an absorber for absorbing solar radiation across virtually the entire surface of the tubular housing; one or more surfaces reflecting solar radiation for indirectly irradiating the part of said absorber which is located on the shadow side of the tubular housing, at least one of the reflecting surfaces extending up to at least the horizontal tangent on the outermost point of the absorber which is located on the side of the housing on which direct solar radiation impinges. Secondly, this invention relates to a mirror comprising one or more reflecting surfaces provided to form part of a solar collector according to the invention.

In this patent application, the indirect radiation is the radiation which is reflected by the reflecting surfaces. The direct radiation is the radiation which impinges directly on the solar collector, in particular the absorber.

Conventional energy sources are gradually becoming depleted. The use of solar power for heating water or generating electricity offers a solution. Various solar power systems have already been disclosed.

In the past few years, a number of different kinds of collectors have been introduced onto the market, ranging from very expensive systems to more affordable systems for private use.

One of the systems which is becoming increasingly widespread uses solar collectors with vacuum tubes having a round absorber. This is a system with evacuated concentrical tubes collecting solar power based on the DEWAR principle, where an internal glass tube is provided with a spectral-selective layer which is built into a second glass tube, one side of which is connected to another by fusion.

The space between the two tubes is brought to a high vacuum and then serves as insulation in order to prevent absorbed energy from being lost again. In order to provide sufficient radiation surface, collectors of this type essentially consist of tubes lying next to one another in close proximity. However, these collectors have the drawback that the available radiation surface is not used efficiently as the solar radiation only impinges on half of the absorber. Namely, only the top side of the absorber is hit by the direct radiation of the sun. In addition, the space between the tubes is not used either.

hi order to compensate for the lost energy per square metre of radiation surface, many tubes have to be used, which results in an expensive and heavy system.

hi order to collect the radiation between the tubes, it is known to position a mirror underneath the solar collectors. As a result, the solar radiation, in particular the indirect radiation, now also impinges on the bottom of the absorber. Such systems are described, inter alia, in European publication EP 0 061 222 and American publication US 4,059,093. However, a drawback of such systems is that they are not yet sufficiently efficient.

It is the object of the invention to produce a solar collector, in which the absorber is irradiated partly directly and partly indirectly, in such a manner that, when the solar radiation is incident at right angles, the absorber is irradiated over virtually the entire circumference and at the most efficient angle of incidence, resulting in excellent efficiency.

The object of the invention is achieved by providing a solar collector, comprising: a tubular housing which is provided inside the space of the housing with an absorber for absorbing solar radiation across virtually the entire surface of the tubular housing; one or more surfaces reflecting solar radiation for indirectly irradiating the part of said absorber which is located on the shadow side of the tubular housing, at least one of the reflecting surfaces extending up to at least the horizontal tangent on the outermost point of the absorber which is located on the side of the housing on which direct solar radiation impinges; in which the solar collector according to the invention is characterized in that the surfaces reflecting the solar radiation and the absorber are designed such that all solar radiation incident at right angles impinges on the absorber, on the one hand directly and on the other hand by reflection on the reflecting surfaces, in such a manner that the sine of the angle of incidence of said radiation on the absorber is substantially equal to 1.

Apart from the fact that the absorber is partly directly irradiated, this solar collector has the considerable advantage that the other parts of the absorber, i.e. both the parts that are situated above and below (shadow side) the horizontal axis of the absorber, are also indirectly irradiated. As no radiation is lost between the tubes and more radiation impinges on the absorber, fewer solar collectors will be required in order to form an array of solar collectors capable of results equal to those of the known systems. As a result of the specific design of the surfaces reflecting the solar radiation on the one hand and the absorber on the other hand, the direct radiation incident at right angles will hit the absorber at right angles (at an angle of 90°) and the indirect radiation will likewise hit the absorber at right angles. This results in an absorbance of virtually 100% of the available solar radiation being achieved in solar collectors of this kind, as the sine of the angle of incidence of the radiation impinging on the absorber is substantially equal to 1.

According to a first preferred embodiment of the solar collector according to the invention, the solar collector comprises a flat first and second reflecting surface which are in each case situated on one side of the vertical axis of the absorber, respectively, and said absorber is cruciform as a result of four flat ribs, each of which is provided virtually in the centre on a side of a square central section. As a result of the arrangement of the reflecting surfaces, both the complete left-hand and right- hand lateral side of the absorber are irradiated indirectly. The specific shape of the absorber will ensure that the solar radiation incident at right angles will impinge on the absorber partly directly and partly through reflection on the reflecting surfaces (indirect radiation) at right angles to the absorber. Consequently, with such a design, an absorbance of virtually 100% (sin 90°= 1) of - A - the available solar radiation is achieved over the width and length of the reflecting surfaces.

In a particular embodiment of the solar collector according to the invention, the first and second reflecting surface are positioned at an angle of 45° relative to the vertical axis of the absorber, as a result of which an angle of 90° is formed between said reflecting surfaces, and the first and/or second reflecting surface are designed as a V- shape, with the legs of the V forming an angle of 90°. More particularly, the vertex of the V of the first and/or second reflecting surface is situated on a vertical line between the wall of the housing of the absorber and the outermost point of the horizontal ribs of the absorber. By constructing the reflecting surfaces in this manner, it becomes possible to increase the supply of solar radiation to the absorber (larger radiation surface) through also irradiating the bottom (is also on the shadow side of the housing) of the absorber at the most efficient angle of incidence when the radiation is incident at right angles.

According to a second preferred embodiment of the solar collector according to the invention, the solar collector comprises a first and a second reflecting surface which are of at least partly concave design and each of which is respectively located on one side of the vertical axis of the absorber, and said absorber comprises a semicircular section. Solar collectors provided with such concave reflecting surfaces in combination with a semicircular absorber also yield very good results. With this design, the part of the indirect radiation will be directed towards virtually the centre of the absorber on account of the reflecting surfaces when solar radiation is incident at right angles and thus hit the absorber at the most efficient angle of incidence (sin 1). The top (horizontal) part will be irradiated by direct radiation with an absorber of such design. Thereby, an absorbance of virtually 100% (sin 90°=l) of the available solar radiation is achieved over the width and length of the reflecting surfaces with such a design.

More particularly, said absorber consists of a semicircular section and a flat rib positioned virtually in the centre of the semicircular section and having a length corresponding to half the diameter of the absorber. The specific shape of the absorber will ensure that the solar radiation incident at right angles will either impinge on the absorber directly or through reflection on the reflecting surfaces (indirect radiation) at right angles to the absorber. Consequently, with such a design, an absorbance of virtually 100% (sin 90°= 1) is also achieved but with a larger radiation surface.

In a particularly advantageous embodiment of the solar collector according to the invention, a reflecting surface comprises the following parts, the length of these parts being expressed relative to the diameter X of the absorber: a flat part (M7) having a length of 0.9427 x X from the point of contact (P) of the line which starts at distance 1.8416 x X (Z) from the horizontal axis of the absorber at an angle of 45° relative to the vertical axis of the absorber to the point of contact (P) with the horizontal tangent of the top of the absorber; a part of a circle (M8) having a radius of 5.7091 x X, and having a centre (Ml) situated at a distance of 1.8808 x X in a direction relative to the horizontal centre axis opposite the spot where the part of the circle (M8) is situated, and a distance of 0.3111 x X in a direction relative to the vertical centre axis opposite the spot where the part of the circle (M8) is situated; a part of a circle (M9) having a radius of 5.2910 x X, and having a centre (M2) situated at a distance of 1.6924 x X in a direction relative to the horizontal centre axis opposite the spot where the part of the circle (M9) is situated, and a distance of 0.2250 x X in a direction relative to the vertical centre axis opposite the spot where the part of the circle (M9) is situated; a part of a circle (MlO) having a radius of 0.8068 x X, and having a centre (M 15) which is the point of intersection between the 45° line which starts from the centre of the absorber and the outer wall of the absorber, and situated at a distance of 0.6376 x X in a direction relative to the vertical and the horizontal centre axis identical with the spot where the part of the circle (MlO) is situated; a part of a circle (M 14) having a radius which corresponds to the radius of the outer wall of the tubular housing + lmm, and having a centre which is situated in the centre of the absorber.

In a preferred embodiment of the solar collector according to the invention, said absorber is made from a thermally conductive material on which a spectral-selective layer is disposed, and is situated at a distance from the housing, in such a manner that the absorber can expand at elevated temperature. More particularly, the absorber is provided with an opening for heat emission of the heat absorbed from the solar radiation.

In a more particular embodiment of the solar collector according to the invention, the tubular housing is of single-walled design. By designing the housing with a single wall, less radiation is lost by reflection on glass walls (namely only one wall present) and on radiation surface as the absorber can be positioned closer to the wall.

With another embodiment of the solar collector according to the invention, the tubular housing is of double-walled design. There is only a vacuum in the intermediate space (Dewar) and it is particularly advantageous to place an absorber in housings of this type in connection with insulation.

In a particularly advantageous embodiment of the solar collector, the absorber is provided with photovoltaic cells in order to convert the absorbed solar radiation into electricity.

In a most preferred embodiment of the solar collector according to the invention, said solar collector is a vacuum solar collector.

In addition, this invention also relates to an array comprising at least two solar collectors according to one of the preceding claims, the reflecting surfaces forming one continuous surface.

Another subject-matter of this application relates to a mirror comprising one or more reflecting surfaces, said mirror being provided to form part of a solar collector according to one of Claims 1 to 13.

In the following description, a detailed description of a number of possible embodiments of a solar collector according to the present invention is given with reference to the attached drawings, solely with the intention of explaining and illustrating the features of the invention. This description should therefore in no way be interpreted as a restriction of the scope of protection of this invention which is defined in the claims.

Li this description, reference numerals are used to refer to the attached drawings, in which:

Fig. 1 shows a representation of a solar collector with a double-walled housing and with a flat first and second reflecting surface in combination with a cruciform absorber in which the vertex of the V of the first and second reflecting surface is situated on the vertical tangents (Bl and B2); - Fig. 2 shows a representation of a solar collector with a single-walled housing and with a flat first and second reflecting surface in combination with a cruciform absorber in which the vertex of the V of the first and second reflecting surface is situated on a vertical line (D) situated between the wall within which the absorber is situated and the outermost point of the horizontal ribs of the absorber;

Fig. 3 shows a representation of a solar collector with a double-walled housing and with a flat first and second reflecting surface in combination with a cruciform absorber in which the vertex of the V of the first and second reflecting surface is situated on a vertical line (D) situated between the wall within which the absorber is situated and the outermost point of the horizontal ribs of the absorber;

Fig. 4 shows a representation of a solar collector with a double-walled housing, the reflecting surfaces of which are of concave design and the absorber of which comprises a semicircular section; - Fig. 5 shows a representation of a solar collector with a single-walled housing, the reflecting surfaces of which are of concave design and the absorber of which comprises a semicircular section;

Figs. 6 and 7 show that part of the incident radiation hits the top part directly and indirectly at right angles and is focussed at the bottom in the centre of the absorber;

Fig. 8 shows the construction drawing of a concave reflecting surface;

Figs. 9.1, 9.2, 9.3 and 9.4 show a representation of possible arrays of solar collectors. During the design of the solar collector, the following two principles were taken into account:

1) The intensity of solar radiation is determined by the angle of incidence. In the case of solar radiation, this is expressed in W/m² on a horizontal flat surface. This can be calculated by determining the Sin of the angle of incidence -»

90° = 1 and 0° = 0; Sin x 100 = % of the available solar radiation at perpendicular solar radiation on a horizontal flat surface. Applying this theorem to a round absorber (that is an absorber in which a spectral-selective layer is applied to virtually the entire circumference of the inner tube in its housing), 90° is in this case the top of the tubular housing (2) where the solar radiation (4) is incident at right angles and gradually decreases over 45° to 0 at ¹A diameter. If this calculation is used on ¹A circle per ° with fixed radiation at right angles to the circle, it is clear that 64% of the available solar radiation is absorbed on ¹A circumference of the circle and 100% is absorbed on the flat surface. If, as a result of a mirror, the available solar radiation is equal at the bottom and at the top, this equals 64% across the surface of the circle. As the sun travels in an arc, the tube without mirror will have a constant 64% absorbance of the available solar radiation across ¹A the surface. 2) The law of reflection = the angle of incidence on a reflecting surface is the angle of reflection both with flat surfaces and with curved surfaces.

The solar collector according to the invention and as illustrated in the figures comprises: - a tubular housing (2) which is provided inside the space of the housing (2) with an absorber (3) for absorbing solar radiation (4) across virtually the entire surface of the tubular housing (2); one or more surfaces (5) reflecting solar radiation for indirectly irradiating the part of said absorber (3) which is located on the shadow side of the tubular housing (2), at least one of the reflecting surfaces (5) extending up to at least the horizontal tangent (A) on the outermost point of the absorber (3) which is located on the side of the housing (2) on which direct solar radiation impinges so that the sides are completely irradiated. Depending on its use, said tubular housing (2) is of single-walled or double-walled design, the space between the walls with a double-walled design being between 2 and 30 mm, preferably between 5 and 20 mm, and more particularly 10 mm.

The surfaces reflecting solar radiation can be made from any material reflecting radiation, and by positioning the reflecting surfaces (5) adjoining one another, it is possible to produce an array (see Fig. 9).

The best results are achieved with the array illustrated in Fig. 9.1, in which the reflecting surfaces provided in each solar collector extend up to at least the horizontal tangent on the outermost point of the absorber which is located on the side of the housing on which direct solar radiation impinges. Slightly reduced, but still good results are achieved with the array of solar collectors illustrated in Figs. 9.2 and 9.3, where the solar collectors which form part of the array are positioned closer to one another. In this case only the reflecting surfaces which are situated on the ends of the array extend to the horizontal tangent on the outermost point of the absorber of the respective solar collector (1). The array may also consist of, as illustrated in Fig. 9.4, a sequence of concave mirrors, as a result of which the incident solar radiation is indirectly focussed in the centre of the absorber.

It is an object of this invention to provide a solar collector (1) which is as efficient as possible, that is to say it is an object to achieve an absorbance of virtually 100% of the available solar radiation incident at right angles over the width and length of the solar collector. This is achieved by designing the surfaces (5) reflecting the solar radiation and the absorber (3) in such a manner that all solar radiation (4) incident at right angles impinges on the absorber (3), on the one hand directly and on the other hand by being reflected on the reflecting surfaces (5), in such a manner that the sine of the angle of incidence of said radiation on the absorber (3) is substantially equal to 1.

Such a result can be achieved using a solar collector (1) (illustrated in Figs. 1 to 3) which comprises a flat first and second reflecting surface (5), which are in each case situated on one side of the vertical axis (C) of the absorber (3), respectively, and in which said absorber (3) is cruciform as a result of four flat ribs (8), each of which is provided virtually in the centre on a side of a square central section (6), also referred to as cruciform absorbers.

The first and second reflecting surface (5) are arranged at an angle of 45° relative to the vertical axis (C) of the absorber (3), as a result of which an angle of 90° is formed between said reflecting surfaces (5), and that the first and/or second reflecting surface (5) are designed as a V-shape, with the legs of the V forming an angle of 90°.

By constructing the first (5a) and second (5b) reflecting surface in the form of a V- shape, with the legs (5al and 5a2; 5bl and 5b2) of the V forming an angle of 90° and with the vertical tangents (Bl and B2) coinciding exactly with the vertex. Consequently, with radiation incident at right angles, the cruciform absorber (3) will also be irradiated over its entire lateral sides and also on its bottom side at the location between the two tangents (Bl and B2), in other words the locations where previously Sin = 0, it is now true that Sin = 1. With such an arrangement, the absorbance of the horizontal radiation surface will increase. The results can further be improved by providing the solar collector or the array with a tracking system so that the solar radiation impinges on the absorber and the reflecting surfaces continuously at right angles.

By providing the solar collector (1) with an absorber (3) which is flat rather than round, the radiation can impinge on such absorbers at right angles and an absorbance of 64% therefore increases to virtually 100% (Sin 90°=l). Such absorbers (3), also referred to as cruciform absorbers, are made of a thermally conductive material on which a spectral-selective layer is disposed, and are situated at a distance from the housing (2), in such a manner that the absorber (3) can expand at elevated temperature and is furthermore provided with a central section (6) provided with an opening (7) for heat emission of the heat absorbed from the solar radiation. For this purpose, the opening (7) is preferably provided with, for example, a "heat pipe", U- pipe, flow-through element or any other possible embodiment for transferring the energy obtained. This absorber (3) is cruciform as a result of four flat ribs (8) which are provided on a central section (6). The central section (6) preferably has a square shape in order to also absorb virtually 100% of the radiation incident at right angles. The cruciform absorber is made of aluminium or another thermally conductive material and may be placed under a slight vacuum in order to protect it against oxidation, on the one hand, and in order to insulate it against the outside, on the other hand. As illustrated in Figs. 1 and 2, such absorbers can be placed both in a single- walled and in a double-walled housing (2). However, care should be taken to ensure there is sufficient distance between the ribs of the cruciform absorber and the glass tube, in order to maintain the insulating action of the vacuum, particularly with the single-walled design. With the double-walled design, providing sufficient distance between the ribs of the cruciform absorber and the glass tube is important in order to allow for expansion.

With such cruciform absorbers, virtually 100% absorbance (Sin 1) of the available solar radiation is achieved with solar radiation incident at right angles on the top and bottom, horizontal ribs and lateral sides of the vertical ribs (8), as well as the four sides of the central section (6). There is a slight loss due to reflection on the glass of the tubular housing (2) and the space between the housing (2) and absorber (3). The virtually 100% absorbance of the available solar radiation can be maintained if the reflecting surfaces (5) and the absorber (3) in its housing of the solar collector (1) or in array rotate with the sun (tracking).

A slight modification is still required as it is not certain that the radiation between the cruciform absorber and the glass tube goes through to the reflecting surface (as a result of the reflection on the glass of the housing). Therefore, and as illustrated in Fig. 3, the vertex of the V is on a vertical line (D) situated between the wall within which the absorber (3) is situated and the outermost point of the horizontal ribs (8).

A good result is already achieved with the above-described embodiments (with flat reflecting surfaces), but this can still be improved with respect to the angle of incidence of the radiation on a preferably round absorber (3).

If it is intended to improve the angle of incidence with round absorbers, this can only be done for the bottom half (part situated below the horizontal axis), that is to say on the reflecting surface (5). This can only be achieved by directing the radiation on the reflecting surface (indirect radiation) towards the centre of the round absorber (3) (Sin 1) by making the reflecting surfaces of concave design (see Figs. 4 to 8). First, the two corners at the bottom of the reflecting surface are rounded off and the reflecting surface is then bent from Vz e of the absorber (3) in such a manner that the solar radiation is focussed. In order to focus the radiation as much as possible on the centre of the absorber when the solar radiation is incident at right angles, the concave reflecting surface (see in particular Fig. 8) is composed of different parts, the length of these parts being expressed relative to the diameter X of the absorber: a fiat part (M7) having a length of 0.9427 x X from the point of contact (P) of the line which starts at distance 1.8416 x X (Z) from the horizontal axis of the absorber at an angle of 45° relative to the vertical axis of the absorber to the point of contact (P) with the horizontal tangent of the top of the absorber; a part of a circle (M8) having a radius of 5.7091 x X, and having a centre (Ml) situated at a distance of 1.8808 x X in a direction relative to the horizontal centre axis opposite the spot where the part of the circle (M8) is situated, and a distance of 0.3111 x X in a direction relative to the vertical centre axis opposite the spot where the part of the circle (M8) is situated; a part of a circle (M9) having a radius of 5.2910 x X, and having a centre (M2) situated at a distance of 1.6924 x X in a direction relative to the horizontal centre axis opposite the spot where the part of the circle (M9) is situated, and a distance of 0.2250 x X in a direction relative to the vertical centre axis opposite the spot where the part of the circle (M9) is situated; a part of a circle (MlO) having a radius of 0.8068 x X, and having a centre (Ml 5) which is the point of intersection between the 45° line which starts from the centre of the absorber and the outer wall of the absorber, and situated at a distance of 0.6376 x X in a direction relative to the vertical and the horizontal centre axis identical with the spot where the part of the circle (MlO) is situated; a part of a circle (M14) having a radius which corresponds to the radius of the outer wall of the tubular housing + lmm, and having a centre which is situated in the centre of the absorber.

The surface (M7) partly runs along the horizontal centre axis of the absorber in order to keep the indirect radiation on the absorber at that point as close to Sin 1 as possible. The surface (MlO) is the rounding of the centring main surfaces M8 & M9 to wards the absorber and also serves to bring as much of the radiation, other than that at right angles to the reflecting surface, onto the absorber.

As a result, a curve of 45° becomes possible for the solar position to the left and to the right of absorber. Connection (M 14) between reflecting surface to the left and to the right on which the housing is situated.

The shape is formed by blending the tangents MlO, M9, M8, M7 with one another. If these measurements or centre points of the shape are displaced or changed, the focus will no longer be at its maximum in the centre of the absorber (Sin 1). If the reflecting surfaces are produced separately, an overlap will be provided.

In order to arrive at an absorbance of virtually 100%, the shape of the absorber (3) also has to be modified. The absorber (3) may be produced by extrusion in convex or concave form or by folding any thermally conductive material, on which a thermally absorbing layer is provided.

As has already been stated, the absorber (3) preferably comprises a semicircular section (3.1) if the reflecting surface is concave and the absorber (3) is preferably flat (cruciform absorber) if the reflecting surface (5) is flat in order to achieve a Sin of substantially 1 for the angle of incidence of the radiation.

Solar collectors provided with such concave reflecting surfaces in combination with a semicircular absorber (3.1) (see in particular Figs. 4 and 5) achieve a good result. With this design, on account of the reflecting surfaces, the part of the indirect radiation will be directed towards virtually the centre of the absorber and thus impinge on the absorber at the most efficient angle of incidence (sin 1) when the solar radiation is incident at right angles. With an absorber of this design, the top (horizontal) part will be irradiated by direct radiation. Consequently, with such a design, an absorbance of virtually 100% (sin 90°= 1) of the available solar radiation is achieved over the width and length of the reflecting surfaces.

As the top of the concave reflecting surfaces consists of flat surfaces at an angle of 45° and a sin 1 is desired, the top of the absorber will have to be vertically flat. Thus, as illustrated in Figs. 6 and 7, bottom half circle to 54 (3.1) and top flat vertically directed rib (3.2). Using this reflecting surface (larger radiation surface) and such an absorber, an absorbance of virtually 100% is the available solar radiation is achieved over the width of the reflecting surface. It is possible to modify the scale; with larger or smaller absorbers and housings, the scale of the reflecting surfaces has to be modified accordingly.

As illustrated in Figs. 6 and 7, the absorber (3) consists of a semicircular section (3.1) and a flat rib (3.2) positioned virtually in the centre of the semicircular section having a length corresponding to half the diameter of the absorber. The specific shape of the absorber will ensure that the solar radiation incident at right angles will either impinge on the absorber directly or through reflection on the reflecting surfaces (indirect radiation) at right angles to the absorber. Consequently, with such a design, an absorbance of virtually 100% of the available solar radiation is achieved (sin 90°= 1).

If the space between the absorber (3) and the outer tube has to be adjusted in order to achieve sufficient insulation through the underpressure, the difference will be compensated for by the vertical height of MlO in such a manner that the indirect radiation will remain focussed on the centre of the absorber.

All the embodiments described above can also be used to produce a system in which the absorbed solar radiation is converted into electricity. To this end, the absorber (3) has to be in the form of photovoltaic cells or a coating therefor. In that case, there will be a two-point connection rather than, for example, a heat pipe in order to collect and centralize the electricity.

Finally, it should be noted that the reflecting surfaces (5) described, in particular those of flat design, such as illustrated in Figs. 1 to 3, and those of concave design, such as illustrated in Figs. 4 to 8, may be provided on various housings (2): - on the existing vacuum tubes;

- double-walled vacuum tube with modified absorber;

- single-walled vacuum tube with modified absorber.

Claims

1. Solar collector (1), comprising: a tubular housing (2) which is provided inside the space of the housing (2) with an absorber (3) for absorbing solar radiation (4) across virtually the entire surface of the tubular housing (2); one or more surfaces (5) reflecting solar radiation for indirectly irradiating the part of said absorber (3) which is located on the shadow side of the tubular housing (2), at least one of the reflecting surfaces (5) extending up to at least the horizontal tangent (A) on the outermost point of the absorber (3) which is located on the side of the housing (2) on which direct solar radiation impinges; characterized in that the surfaces (5) reflecting the solar radiation and the absorber (3) are designed such that all solar radiation (4) incident at right angles impinges on the absorber (3), on the one hand directly and on the other hand by reflection on the reflecting surfaces (5), in such a manner that the sine of the angle of incidence of said radiation on the absorber (3) is substantially equal to 1.

2. Solar collector (1) according to Claim 1, characterized in that the solar collector comprises a flat first and second reflecting surface (5) which are in each case situated on one side of the vertical axis (C) of the absorber (3), respectively, and in that said absorber (3) is cruciform as a result of four flat ribs, each of which is provided virtually in the centre on a side of a square central section (6).

3. Solar collector (1) according to Claim 2, characterized in that the first and second reflecting surface (5) are positioned at an angle of 45° relative to the vertical axis (C) of the absorber (3), as a result of which an angle of 90° is formed between said reflecting surfaces (5), and in that the first and/or second reflecting surface (5) are designed as a V-shape, with the legs of the V forming an angle of 90°.

4. Solar collector (1) according to Claim 3, characterized in that the vertex of the V of the first and/or second reflecting surface (5) is on a vertical line (D) situated between the wall of the housing of the absorber (3) and the outermost point of the horizontal ribs of the absorber (3).

5. Solar collector (1) according to Claim 1, characterized in that the solar collector (1) comprises a first and a second reflecting surface (5) which are of at least partly concave design and each of which is respectively located on one side of the vertical axis (C) of the absorber (3), and in that said absorber comprises a semicircular section.

6. Solar collector (1) according to Claim 5, characterized in that said absorber (3) consists of a semicircular section and a flat rib positioned virtually in the centre of the semicircular section and having a length corresponding to half the diameter of the absorber (3).

7. Solar collector (1) according to Claim 5 or 6, characterized in that a reflecting surface (5) comprises the following parts, the length of these parts being expressed relative to the diameter X of the absorber: a flat part (M7) having a length of 0.9427 x X from the point of contact (P) of the line which starts at distance 1.8416 x X (Z) from the horizontal axis of the absorber at an angle of 45° relative to the vertical axis of the absorber to the point of contact (P) with the horizontal tangent of the top of the absorber; a part of a circle (M8) having a radius of 5.7091 x X, and having a centre (Ml) situated at a distance of 1.8808 x X in a direction relative to the horizontal centre axis opposite the spot where the part of the circle (M8) is situated, and a distance of 0.3111 x X in a direction relative to the vertical centre axis opposite the spot where the part of the circle (M8) is situated; a part of a circle (M9) having a radius of 5.2910 x X, and having a centre (M2) situated at a distance of 1.6924 x X in a direction relative to the horizontal centre axis opposite the spot where the part of the circle (M9) is situated, and a distance of 0.2250 x X in a direction relative to the vertical centre axis opposite the spot where the part of the circle (M9) is situated; a part of a circle (MlO) having a radius of 0.8068 x X, and having a centre (M15) which is the point of intersection between the 45° line which starts from the centre of the absorber and the outer wall of the absorber, and situated at a distance of 0.6376 x X in a direction relative to the vertical and the horizontal centre axis identical with the spot where the part of the circle (MlO) is situated; a part of a circle (M14) having a radius which corresponds to the radius of the outer wall of the tubular housing + lmm, and having a centre which is situated in the centre of the absorber.

8. Solar collector (1) according to one of the preceding claims, characterized in that said absorber (3) is made from a thermally conductive material on which a spectral-selective layer is disposed, and is situated at a distance from the housing (2), in such a manner that the absorber (3) can expand at elevated temperature.

9. Solar collector (1) according to one of the preceding claims, characterized in that the absorber (3) is provided with an opening (7) for heat emission of the heat absorbed from the solar radiation.

10. Solar collector (1) according to one of the preceding claims, characterized in that the tubular housing (2) is of single-walled design.

11. Solar collector (1) according to one of Claims 1 to 9, characterized in that the tubular housing (2) is of double- walled design.

12. Solar collector (1) according to one of the preceding claims, characterized in that the absorber (3) is provided with photovoltaic cells in order to convert the absorbed solar radiation into electricity.

13. Solar collector (1) according to one of the preceding claims, characterized in that said collector (1) is a vacuum solar collector.

14. Array (10) comprising at least two solar collectors (1) according to one of the preceding claims, the reflecting surfaces (5) forming one continuous surface.

15. Mirror comprising one or more reflecting surfaces, characterized in that said mirror is provided to form part of a solar collector according to one of Claims 1 to 13.