DE2738667A1 - Light trap for solar energy absorber - has adjacent reflectors with base surfaces at acute angles to incidence of rays - Google Patents

Abstract

The absorber consists of a body in heat-conductive contact with a heat carrier. It has an associated light trap, which consists of a heat insulating material, and has a reflecting surface. The trap is positioned in front of the absorber (28) in the path of the solar rays, and has an aperture (31) to allow their passage. Reflective surfaces (34) and apertures are arranged so that the rays which reach the surfaces are conducted onto the absorber. The light trap may consist of several reflectors positioned next to each other. Each has a base surface directed towards the absorber, and positioned at an acute angle to the direction of incidence of the solar rays.

Description

Absorber for absorbing radiation energy

and its conversion into thermal energy. The invention relates to an absorber to absorb radiation energy and convert it into thermal energy and for forwarding the thermal energy to a heat transfer medium, consisting of a standing in thermally conductive contact with the heat carrier and exposable to the rays Absorber carrier.

Solar collectors are an important field of application for such absorbers for storing and using solar energy. These collectors generally exist from one at the same time the carrier for a heat transfer medium flowing through forming absorber, which is at least partially covered with a translucent shell is.

In the case of the so-called flat-plate collectors, this shell consists of one Glass pane that is placed on a trough-shaped support in which a plate-shaped Absorber is arranged.

Another type of collector is the high-performance collector devices that focus the sun's rays, as well as lens or reflector systems. A well-known collector of this type is the so-called parabolic solar collector, which has a reflector designed as a parabolic cylinder mirror, in its focal line a tubular absorber is arranged, in which the heat carrier flows and the is generally surrounded by a translucent second tube.

In both cases, the absorber is used to absorb the sun's rays, to convert it into thermal energy and to pass it on to the heat carrier, which is then transferred to Line systems to the destination, such as apartment heating or with the thermal energy to be driven power plants is passed on.

In practice, however, the absorbers do not perform their function without loss. This is essentially because the layer of the absorber body on which the Rays impinge and in which the conversion of radiant energy into heat takes place, usually a surface temperature that is considerably higher than the ambient temperature Has. This will cause heat losses by heat radiation to the Environment and in the presence of a fluid outside the absorber (e.g. air) also caused by convective heat transfer to this fluid.

The invention is based on the object by manufacturing technology simple measures to increase the efficiency of an absorber of the type mentioned above Optimization of the energy use of the rays directed at the absorber to improve.

According to a first embodiment, the object is achieved according to the invention solved that the absorber made of a heat insulating material, mirrored Light trap having surfaces is assigned that is in the beam path in front of the absorber body is arranged and which has at least one diaphragm for the passage of the rays, and that the mirror surfaces and diaphragm are directed so that the on the mirror surfaces incident radiation are directed onto the absorber body.

This ensures that the radiation directed onto the absorber can enter through the diaphragm, but that of the hot surface of the absorber body outgoing diffuse heat radiation cannot escape for the most part. Just one a small portion can be lost to the environment through the aperture. The inventive Execution therefore allows the energy-converting surface of the absorber body to be made large without causing large heat losses, so that at the same time the heat transport from the absorber body to the heat carrier also with lower temperature range is carried out. Another advantage of the device is that scattered radiation is also captured by the mirror surfaces will.

The light trap can be in a simple design with a single Aperture should be provided by a narrowing in the central area of the light trap opening is formed, the opening widening on both sides, wide mouths forming, is shaped, in the manner of two funnels, if the light trap is round is. This version is particularly suitable when using point-to-point or line-shaped focal point of bundled rays.

The light trap can also be designed with several apertures, to capture broad bundles of rays. There is an advantageous embodiment for this in that the light trap consists of several mirror bodies arranged next to one another exists, each having a base surface directed towards the absorber body and opposite the direction of incidence of the rays tapering approximately to a point, between the side by side standing base surfaces aperture-forming column are provided.

The major part of the incident rays is thereby on the Directed absorber surface. A small part is in the form of mirror bodies Absorbs heat and causes it Heating up to elevated temperatures. However, this ensures that the diffuse heat radiation from the absorber body surface remains reduced to a minimum, as only the heat escaping through the panels radiation can leave the absorber.

The mirror bodies can be held together by thin frames will. The surface of the ribs are advantageously mirrored to the ones on them to be able to direct incident rays onto the collector body as well.

By placing the mirror body directly on the absorber body the conversion area and thus the surface radiating heat loss is reduced. In this case, the absorber has a toothed surface, with the teeth either arise by appropriate shaping of the absorber or by placing the aforementioned mirror body can come about. In both cases the teeth are flanks are preferably mirrored so that they direct the rays onto the tooth base reflect. The tooth base forms the conversion layer.

This ensures that, if necessary, bundled with a primary system Rays are bundled again and thus the radiation with increased concentration hits the energy-converting surface. This design is particularly suitable for systems in which energy conversion is achieved at high temperatures.

A further improvement in the efficiency of an absorber can can be achieved in that the or more absorber body in an insulating body are embedded, which is toothed on the radiation incidence side. The tooth flanks are mirrored similar to the previous case. The absorber bodies are here up to a small conversion surface that protrudes between the teeth, with insulation surround.

The above object can according to a further embodiment be achieved according to the invention in that the absorber body of the in a translucent Carrier-led heat transfer medium is completely washed around.

A reduction in heat radiation losses is achieved with this design caused by the fact that the absorber body by appropriate shaping with a very large exothermic surface can be formed. Beneficial Versions for this are in the form of corrugated sheet metal or in the form of metal or other chips Absorber body. In this embodiment according to the invention, a good one is achieved Heat dissipation from the absorber body to the heat transfer medium surrounding it, so that heat is radiated to the environment occurs only to a small extent.

It is also possible to prevent heat losses from the absorber with an insulating body to reduce the amount of the incident rays turned away absorber body sides surrounds and its surfaces, which border on the area of the incident rays, mirrored are.

This ensures that the did not participate in the radiation absorption The absorber surface is no longer in unhindered heat exchange with the considerably colder environment. The insulating body takes on the absorber tube during operation facing surface to high temperature values, whereby a heat radiation of the Surface of the absorber body is greatly reduced. In addition, the Mirroring of the insulating body, scattered rays are also captured.

It is advantageous if the insulating body with play around the absorber body arranged and this room is largely closed off with the environment. One Fluid flow between the absorber body and the insulating body is largely reduced as a result prevented, so that a convective heat dissipation of the absorber in this environment is almost completely avoided. The heat build-up in this space becomes accordingly caused by the thermal conductivity of the insulating body. Suitable materials for The insulating bodies are ceramic, mineral or artificial building materials, in the form of foam, granules, fibers or foils as well as their combination.

If the requirements are very high, it goes without saying possible, to combine the various measures according to the invention in order to thus achieve an optimal one Solution to achieve a minimum heat energy loss and a maximum Efficiency allowed. The measures according to the invention also make it possible individually or in combination a free design of the absorber body, so that this the can be optimally adapted to various requirements and purposes. For example, the absorber body can be a metal tube in which the heat transfer medium flows. This tube can either be cylindrical or flattened in the longitudinal direction be designed or also with a, for example, wave-like folded Wall sector.

But it is also possible for the tube to be made of translucent material to produce and to provide half of the inner surface with an absorption layer, which produces the energy conversion.

Embodiments according to the invention are shown schematically in the drawing shown. 1 shows the arrangement of an absorber in a solar collector and FIGS. 2-11 each show an exemplary embodiment.

In Fig. 1, an absorber 10 for using solar energy is in one Parabolic collector shown. An absorber body 11 designed as a tube is attached to this Purpose arranged in the focal line of a semi-cylindrical parabolic mirror 12. the Sun rays 13 reflected by the mirror 12 concentrate in the area of the absorber tube 11, which converts the radiation energy into thermal energy and on a heat transfer medium 14 flowing inside the absorber tube 11 emits. The absorber pipe 11 is surrounded by a glass envelope 15.

An enlarged cross section of the absorber 10 is shown in greater detail in FIG shown. There is a light trap in the area of the incident reflection rays consisting of several conical mirror strips 19 in the annular space between the glass envelope 15 and the absorber tube 11 are arranged. The ones with the base facing the absorber tube conical strips are with frames 20 either on the glass gall or on the absorber tube attached.

The ribs also have a triangular cross-section, like it from Fig. 3 and 4 can be seen, which each have a section along the line III - III or IV - IV represent. The flanks 21 of the mirror strips as well as the santen are mirrored.

in order to deflect scattered radiation onto the absorber 11. Gap between the conical strips are provided as apertures 22 for the entry of the light rays.

The bundled light radiation entering through the cladding tube 15 meets partly on the conical strips 19 and partly through the slit diaphragms 22 the absorber surface 23. On this surface, the so-called conversion layer the conversion of radiant energy into heat takes place, which then takes place over the pipe wall is transferred to the heat carrier 14. Due to the temperatures that occur and the heat flow established by the material of the absorber 11 is sufficient in many cases not enough to lower the temperature at the conversion layer 23 sufficiently, so that heat can be radiated to the environment. This diffuse thermal radiation however, the strips 19 made of a heat-insulating material held back because these are also heated up by the incident rays are. Only a small proportion escapes through the slot gap 22 to the environment. The strips 19 and the frames 20 and 20 'are made of heat-insulating material, to keep the heat conduction in them small.

The light fall for the absorber can, for example, also come from a Diaphragm exist, as shown in Fig. 5.

The absorber body is in this case as a flattened tube 28 formed and shown in cross section. There is a long insulating body for the light trap 29 is provided, which is connected to the absorber body 28 and in the Area 30 of the beam path also runs along the Absorber body 28 guided slot gap to form a diaphragm 31 has. From aperture 31 the gap widens into an opening 32, which on the absorber body 28 is the entire Width this body occupies. The side facing the rays is also im Cross-section formed into a funnel-shaped opening 33.

This design is particularly suitable for a case in which the rays are one-sided and concentrated on the absorber through reflectors or lenses 28, the line-shaped focal point falling on the diaphragm 31.

Scattered rays can also be captured in this design by mirroring the flanks of the insulating body 29 around the opening 32, 33 are, so that radiation incident thereon is deflected onto the absorber 28 can.

In this case, too, the heat losses caused by radiation from the conversion layer 35 of the absorber, which may be strongly heated be reduced. The surfaces of the absorber 28 that are not in contact with radiation are surrounded by an insulating body 36 in order to dissipate heat to the surroundings, be it by radiation or convection, to be avoided as far as possible. In execution According to FIG. 5, the insulating body 36 can be used for the rear side of the absorber body 28 and the elements 29 for the light trap are advantageously made from one component.

The embodiments described above are just one example of one Variety of design options, depending on the shape of the absorber and can be adapted to requirements as required. For example, the light trap be manufactured according to, in which the heat carrier in a translucent body is performed and the absorber body is arranged in the fluid flow. An example this is shown in Fig. 6, in which the heat transfer medium 14 in a permeable Material, e.g. glass, made tube 40 is guided.

The mirror strips 19 'can be here in a simple manner directly applied to the translucent tube 40, e.g.

be glued on, or be formed directly from the tube 40, as shown in the left half of FIG.

The flow cross section of the fluid 14 can either by corresponding Shaping the tube or in the case of a tube 40 with a circular cross-section a filler body 42 can be optimized.

In the embodiments described so far, there is a loss of energy Reduced by measures that heat exchange between the energy converting Layer with the environment prevented. But it is also possible to lose energy due to increased heat transport between absorber and heat transfer medium. In In these cases, the heat transfer medium is fed into a translucent body and within that body put in contact with the absorber body. The advantage of this design is that the absorber body with very large heat transfer surface and also completely from the heat transfer medium can be washed around. The absorber body can be used as chips 43 of a highly thermally conductive Material, e.g. metal, as shown in Fig. 6 on the left half is shown. By appropriate shaping or multiple folding can be one-piece or multi-piece absorber body with a small irradiation cross-section but form a large heat transfer surface. As an example is in Fig. 6, right Half, a zigzag folded sheet metal 44 is shown.

A translucent pipe for the heat transfer medium also offers the possibility of the energy conversion layer by one on one side of the inner wall of the pipe 50 applied absorption layer 51 to form, FIG. 7. The poorly thermally conductive Pipe wall on which the conversion layer 51 is applied serves at the same time as isolation from the environment. An additional good heat transfer from the conversion layer 51 to the heat transfer medium offers one in this embodiment improved efficiency. It is particularly suitable for very flat absorbers, as they are used e.g. for solar flat-plate collectors.

It is of course also possible with opaque ones Absorber tubes the largest possible contact area with the heat transfer medium through appropriate shaping to be generated with the same irradiation area. An example of this is in Fig. 8 shown, in which the radiation facing wall 52 of a flat absorber tube 53 is wavy.

The example shown in Fig. 1 of a so-called parabolic collector is designed so that the light rays strike the absorber 10 on all sides, be it directly or by reflection from the mirror 12. There are other cases when which radiation is either parallel (direct solar radiation) or through lenses can only come in bundled from one direction. In these cases the invention provides a measure before, on its own or in combination with the one previously described Measures can be brought into use. This measure consists in getting through a heat-insulating body to generate a build-up of heat on the sides of the absorber, that are not exposed to radiation. The insulating body can either, as in Fig. 5, are in direct contact with the absorber body, or even better below Formation of a space, as shown in FIG. 6, around said surface of the Absorbers are arranged. In both cases this is due to the low thermal conductivity a heat build-up around the absorber caused, which in the second case a convection flow the air in the gap 55 is almost prevented. It is advantageous if this space 55 is completely sealed off from the atmosphere, as shown in the example in Fig. 6 is achieved.

In this way, an air flow in the space 55 comes to a standstill almost completely brought so that a heat exchange with the environment at these points of the absorber can be optimally prevented. The end surfaces 56 of the insulating body 57, which at Adjacent the radiation area can advantageously be mirrored in order to protect the areas on it redirect incident rays onto the absorber.

In FIGS. 9-11, exemplary embodiments are shown in which a secondary reflection to strengthen the bundling of rays and to reduce the heat emitting Absorber surface is effected. In Fig. 9, one is connected to teeth 60 and 61, respectively Absorber tube 62 shown. The teeth have mirrored flanks 63 and 64 and act in a manner similar to the light trap formed from the mirror bodies 19 in Fig. 2. In contrast to the previous case, here is the energy conversion layer of the absorber is restricted to the gap surfaces 65 and 66 between the teeth. The version with the glued-on teeth 65 in the left half of the drawing has the advantage that the teeth can be made of heat-insulating material, and thus the heat loss radiating surface on the light incidence side almost only the gap areas 65 is reduced.

In the right half of the drawing is a simplified one from a manufacturing point of view Shown form in which the absorber tube 62 is formed directly with a tooth profile is.

In Fig. 10 and 11 variants are shown in which several absorber tubes 70 embedded in an insulating body 71 and 72 with mirrored teeth 73 and 74, respectively are such that in each case a predetermined absorber surface 75 between the teeth stands out. This design is effective and simple in terms of production technology Combination of the various measures described above to reduce Heat losses.

The difference between the two embodiments in FIGS. 10 and 11 lies only in the fact that a common glass envelope 76 is present in FIG. 10, while in FIG. 11 individual glass envelopes 77 for the individual absorber tubes 70 are provided.

Depending on the application, one or the other measure or an appropriate combination between them to optimize the efficiency lead, i.e. the diversity and flexibility in the design options give the possibility, in each case of application, by the rays on the absorber to use almost all of the incident energy.

L e r s e i t e

Claims (17)

Claims S absorber for absorbing radiation energy and converting it into thermal energy and transferring the thermal energy to you Heat transfer medium, consisting of one in thermally conductive contact with the heat transfer medium and absorber bodies which can be exposed to radiation, characterized in that the absorber one made of heat-insulating material, having mirrored surfaces Light trap is assigned, which is arranged in the beam path in front of the absorber body (28) and which has at least one diaphragm (31) for the passage of the rays, and that the mirror surfaces (34) and the diaphragm are directed so that the on the mirror surfaces incident radiation is directed onto the absorber body. 2. Absorber according to claim 1, characterized in that the light trap opening (31) in the middle of the Direction of rays one the diaphragm (31) forming constriction, and expanding continuously on both sides opens. 3. Absorber according to claim 1 or 2, characterized in that the Light falls formed from a plurality of mirror bodies (19) arranged next to one another each having a base surface (56) directed towards the absorber body (11) and against the direction of incidence of the rays run out approximately pointed, and that between the adjacent base surfaces panels (22) forming column is provided are. 4. Absorber according to claim 3, characterized in that the mirror body (19) are held in place with thin frames (20), the surface of which is at least partially is mirrored. 5. Absorber according to claim 3, characterized in that the mirror body (60) are applied to the absorber body. 6. Absorber according to claim 3, characterized in that the mirror body (73) are components of an insulating body (71), and that the absorber body (70) is embedded in the insulator. 7. Absorber for absorbing radiation energy and converting it in thermal energy and for forwarding the thermal energy to a heat transfer medium from a standing in thermally conductive contact with the heat carrier and the rays removable absorber body, characterized in that the absorber body (44) from the heat carrier (14) guided in a transparent carrier (40) completely is washed around. 8. Absorber according to claim 7, characterized in that the absorber body (44) is a corrugated sheet metal. 9. Absorber according to claim 7, characterized in that the absorber body (43) is designed in the form of chips or of several flat parts. 10. Absorber for absorbing radiation energy and converting it in thermal energy and for forwarding the thermal energy to a heat transfer medium from a standing in thermally conductive contact with the heat carrier and the rays exposable absorber body, characterized in that the incident rays surrounded remote sides of the absorber body (28) with an insulating body (36) whose surfaces (34) adjoining the region (30) of the incident rays are mirrored are. 11. Absorber according to claim 10, characterized in that between a gas space (55) is provided for the insulating body (57) and the absorber body, which is as far as possible closed off from the environment. 12. Absorber according to claim 10 or 11, characterized in that the insulating body (57) is made of ceramic. 13. Absorber according to claim 10 or 11, characterized in that the insulating body (57) is made of plastic. 14. Absorber according to one of claims 10-13, characterized in that that the side of the absorber body (28) facing the incident rays with a light trap (29, 31-33) according to claim 1 is provided. 15. Absorber according to one of claims 10-14, characterized in that that the absorber body (44) is completely surrounded by the heat transfer medium (14). 16. Absorber according to one of the preceding claims, characterized in that that the absorber body (53) corrugated tubular with at least one wall piece (52) is formed, and that the heat carrier flows in the tube. 17. Absorber according to one of the preceding claims, characterized by using the absorber in a solar collector to generate energy from sun rays.