DE2947584A1 - SOLAR ENERGY COLLECTOR - Google Patents

Description

Solar energy collector

The invention relates to a solar energy collector or a device for capturing solar energy, which or which is designed so that it can be used in a fixed position and still has good trapping efficiency having. In particular, the invention is concerned with. a collector with a heat absorber that is able to at relatively high temperatures as well as with considerable angular deviations of the solar radiation with respect to the normal Exposure to work.

The most widespread planar solar collectors essentially consist of a glazing, the quite a few Placed centimeters in front of a blackened surface, which is most often a sheet of metal, which receives sun rays through the glass pane and warms up as a result. The captured energy is generally via a heat transfer fluid withdrawn, the in the intimately connected to the absorbent surface itohr pipes or between two interconnected parallel sheets, whereby the sheet directed towards the glazing absorbs the incident solar radiation absorbed.

These sensors enable usable trapping efficiencies only at low working temperatures. You can do this Working temperature as the deviation Δ Τ between the mean Temperature of the receiver (practically the arithmetic mean between the inlet temperature and the outlet temperature of the heat transfer fluid) and the outside temperature of the air in contact with the front surface of the collector. These collectors have an efficiency

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from 70 to 80 * with respect to the arriving at the Vorderf1äche the glazing solar energy when the above-defined deviation </ \ T is less than about 1O ⁰ C. However, this efficiency falls rapidly with increasing deviation / \ T and falls below 20 % for a value Δ of the order of magnitude of 60 to 80 C (more or less corresponds to the incident solar radiation intensity).

Various improvements have been made to this base-collector, in order to improve its efficiency at operating temperatures above 5O ⁰ C.

According to a first embodiment, the black absorbent layer is replaced by a special layer, the so-called selective layer, the property of which is to absorb as much as possible the incident solar energy and as far as possible to limit the self-emission of the absorbing surface, which is caused by the absorption of the incident solar radiation is heated. Such collectors have an efficiency for low values of Δ Τ which is even lower than that described above, but a significantly higher efficiency for values of A T in the order of 40 to 80 ^° C. In addition, the operating threshold corresponds to lower values of the incident Solar power.

According to a second embodiment, the first glazing is assigned a second glazing or a special construction in honeycomb design, which is usually made of a very thin transparent material.

Unfortunately, these improvements diminish that remarkable solar energy and do not allow that one interesting trapping efficiencies for values of

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Λ T equal to or greater than 80 ⁰ C receives.

Other solutions have been proposed to obtain adjacent to favorable efficiencies at operating temperatures 100 ⁰ C. These solutions consist in concentrating the solar energy arriving on the absorbing surface through various optical systems such as mirrors or lenses.

To obtain increased concentration factors, on the order of 10 or 100 or more, it is necessary to that the mirrors are moved in such a way that they continuously direct the sun's rays onto that which forms the absorbing surface Reflect target. This creates a field of heliostats that are controlled by a complex and therefore expensive mechanism are.

In order to obtain intermediate concentration factors that enable the use of fixed facilities, it has been proposed that Use lens systems and mirrors that have a substantially linear concentration on one to the generators of the system Execute a parallel pipe.

The concentration effect can be achieved with either lenses and mirrors with a continuous surface or with lenses and mirrors with a discontinuous surface with "Fresnel optics" will. These devices are of course "east-west" oriented and directed against the central position of the sun. In fact, experience has shown that this type of receiver is hardly suitable for higher services, be it because it would be made movable and permanently adjustable, at least in one direction, in such a way that the focalization zone the rays of the sun stay at the place where the absorber tube that collects the calories is arranged; or you have to keep moving the absorber tube when the mirror is fixed. This is due to the fact that the

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Zone of linear concentration shifts transversely when the Sun rays themselves fall transversely with respect to the lens and that it has not previously been possible to design a device, which have a useful degree of efficiency with all possible sources of thought, especially not with diffuse ones Light to design.

The object of the invention is to produce a solid or semi-solid collector which, however, produces a sufficient concentration to achieve useful trapping efficiencies with widely differing incursions, in particular using heat absorbers which are brought to working temperatures between about 60 and 180 ^° C.

The collector according to the invention comprises at least one optical lens with a straight (1 inigen) focussing or Burning zone and at least one cylindrical concave reflective surface of substantially equal width, the arranged parallel to the optical lens between the lens and the focal line of the latter and the received Lii_it from the lens to at least one elongated straight line Target throws, which is arranged between the optical lens and the reflective surface, wherein the longitudinal dimension of the overall arrangement in east-west direction and is arranged in such a way that the optical axis of the Lens and mirror formed system directed towards the central position of the sun during the period of use is.

According to the invention, this arrangement has an overall precise design.

The target consists of a more or less flattened organ of width c = 3a. It can be, for example

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be a circular absorber tube of diameter £, which carries two diametrically arranged ribs of the same width a.

The mirror is an essentially cylindrical parabolic mirror which, at normal incidence, has a focal length f_ such that £ = 0.75_f and the width of which does not exceed 1_ 4jf; In other words, one can say that the (straight) cross-section of the mirror is approximately a parabolic mirror, the circle of curvature C of which has a radius equal to 2i_ at the tip and that its depth is approximately _f.

However, the profile of the mirror can have a curve different from a parabolic, for example a Correspond to a circle, an ellipse or a hyperbole or even facets or a polyhedron, provided that this Profile is closely adjacent to the parabolic defined above; however, the parabolic shape remains the preferred shape.

The target wi rd in front of the mirror by a distance d in the order of magnitude arranged from 0.75 _f.

The arrangement formed in this way is arranged itself behind a Fresnel lens, the front side of which, namely the one directed towards the sun, is preferably flat. The lens itself has a focal length F at normal incidence of the order of magnitude of 5 ^ and, as said, a width L adjacent to the width 1_ of the mirror. In the implementation, it can be a prismatic lens, that is, a lens with flat surfaces or facets, since it is not necessary that an exact image of the sun is provided; a sufficiently concentrated focal spot alone is sufficient, its maximum width; t at normal incidence preferably a value equal to two thirds of the width £ the through

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the target formed by the absorber, i.e., does not exceed twice the width a defined above; after the agreement The focal length of such a lens is the distance between its back and the spot or point mi η im;] 1 denotes width.

This lens is placed in front of the mirror at a distance D substantially equal to 3.5_f. These Data allow certain tolerances as defined below; the design of the lens is explained below.

In summary and more precisely one can say: one takes as The system practically retains the reference value the value a defined above equal to one third of the width of the target same performance within the following range:

1 - 'L - 15a

_f = 4a + 0.2a

c - 5a

d - 3, 2a + 0.2a

F - 19.5a + 1.5a t 4 2a

D = 13.5a + 0.5a

It is: still possible, acceptable solutions with appropriate Try to find a larger or wider area that matches:

f = 4 a _ + 0.5a d = 3a + 0.5a

F = 20a + 5a

t 4

D =

3.5a + a

In practice it is preferred to use several elementary ones Collectors which are the same as the collector described above and which are placed side by side along the long sides of the optical cylindrical lenses and the cylindrical concave mirror arranged by the series containing the heat transfer fluid Pipelines switched in a known manner.

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An example embodiment of the invention is now intended will be explained in more detail with reference to the accompanying drawings. These show in

Fig. 1 is a cross-section through the relative arrangement of the various elements of the collector and shows the Path of a bundle of sunlight falling on a cylinder of the collector at normal incidence;

2 shows the layout of a prismatic Fresnel lens and the arrangement of the associated parabolic mirror;

3 shows a cross section along the line ITI-III of FIG. by a solar collector composed of four elementary collectors; and

Fig. 4 is a plan view of Fig. 3 on a reduced scale, seen from the glazing side, in which the Tubes are visible through the optical lenses due to transparency.

The collector shown includes a Fresnel lens 10 rectangular shape, the flutes 12 of which are oriented in the longitudinal direction of the lens; a parabolic mirror 14 is parallel to the Fresnel lens 10 between the latter and its dividing line and a straight tube 16 with ribs 16a, the of a heat transfer fluid is flowed through, arranged and is stored along the concentration zone of the catadioptric arrangement, the latter due to the assignment of lens and Mirror is formed.

A special design of the Fresnel lens will now be explained with reference to FIG. 2.

The transverse dimension L of the lens 10, the "focal length" F of this Lens at normal incidence and the width t ^ of the concentration zone 18 is given, i.e. the zone in which a pseudo-image of the sun is formed through the lens.

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ι -

Assuming that the outer surface of the lens is flat, one calculates the angle, '/, of the inclined plane surface Ali of the first outer fluting of the lens so that an incident beam S ₁ A, normal to the lens, passes through the point 1 ', the outermost edge of the concentrate ionszone 18 passes. In de ι' practice, it is the height h of the Kannclierung for making the lens in front, whereby the point B is determined. one then draws the surface BC which is perpendicular to the lens plane is; the distance AC is less than the latitude jt.

The construction is then started again, starting from point C, in such a way that the incident beam S "C is deflected and passes through point P." The angle OC ₂ ui ₄ d is thus obtained, thus the point D. You then go to the center of the lens 10 without exceeding the width t_ until the inclined surface of a prism intersects the center plane XX of the lens. The other half of the lens is designed symmetrically with respect to the plane XX. It is advantageous to bring the tips of the central prisms into the plane of the other tips.

This special lens can be used industrially using the technology for rolled flat glass with a number of technological adjustments be made. For example, a slight inclination of the surfaces BC in order to form a taper is introduced; one uses radii of about 0.5 mm in the dihedral angles of the points such as B, C, D. Of course, the maximum thickness is of the lens is also equal to the thickness £ of the glazing base, enlarged by the height h of the fluting 12, which is preferably less than 3 mm. The different Characteristic variables of the system can also be seen from the figure.

According to FIGS. 3 and 4, the solar energy collector comprises on the one hand a glazing 20, which consists of the assignment of several optical elongated lenses is formed, these with their

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- I 2 -

Long sides are connected and on the other hand mirrors that For example, consist of a thin sheet of stainless steel 22, in which one by deep drawing or after any Another equivalent method parabolic cells parallel to each other Profiles that take on the role of mirrors. According to the example chosen, there are four optical Systems arranged parallel to glazing 20 at a distance from the latter which is close to twice the The radius of curvature in the tip of the mirror is such that the planes of symmetry X-X of the latter are those of the lenses correspond.

The collector also has an absorbing surface 24 which, for example, consists of four tubes 16 connected in series exist, which makes it possible to have a single inlet 26 and a single outlet 28 for the heat transfer fluid obtain. Each tube is placed in the median plane of the catadioptric mapping of the lens-mirror. The focal length of the lenses, the radius of the mirrors, and the distance between lenses and mirrors will be as above specified, chosen.

The collector elements are advantageous in the interior of a parallelepiped-shaped housing 30, which is at its base is open, arranged and which has a cross section substantially the same as the optical system defined above. The cell-like surface 22 is above the bottom 32 of the housing with the interposition of an insulating Material 34 arranged, which is intended to limit the heat losses of the mirror 14 during the glazing 20 is established in the open area of the housing. The concavity of the mirror 14 and the fluting 12 of the Fresnel lenses 10 are directed towards the interior of the housing.

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3 -

The housing itself is mounted on an axis 36 on which it is locked after adjusting its inclination.

Its length can advantageously be 3 m and thus correspond to the dimensions of easy-to-use glazing.

The absorber tube with the width c_ can advantageously be treated in such a way that its surface becomes a selective area with low radiation losses.

A small band of glazing of the same width can be placed in front of the surface of the tube facing the lens provide that the losses of the pipe by radiation and convection are reduced.

The tube with ribs can be replaced by a tube of larger diameter without ribs or a flattened tube be. It can also be placed in a cave made of thin glass, thereby increasing the efficiency of the collector is increased when this should work in its highest temperature range. The presence of the shell is for working not favorable at temperatures of the order of 80 ° C.

For example, numerical characteristics of a collector according to the invention are given below.

Focal length of the Fresnel lens Width of the Fresnel lens Prism height

Width of the focal spot Focal length of the parabolic mirror

Distance from the flat surface of the lens to the highest point of the mirror

Diameter of the finned tube Width of the fins Distance of the tube from the mirror

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200 mm 150 mm 2 , 5 mm 10 mm 40 mm 143 mm 10 mm 10 mm 32 mm

It goes without saying that every homothetic system is of this equivalent to this.

In addition, the system keeps practically the same Benefits for changes to

_ + 10 mm of the focal length of the Fresnel lens, + _ 2 mm of the focal length of the mirror, _ + 5 mm at the distance Linsc-Spicgcl, _ + 1 to 2 mm with respect to the position of the finned tube.

With such a solar energy collector, the geometric efficiency, which is defined as the part of the lens surface whose exiting rays are captured by the absorber tube, is greater than 70 % and often greater than 90 % for incidents per hour or longitudinal incidence of + _ 40 ° (corresponding to a Duration of _ + 2 hours 40 minutes), but only of ± _ 10 ° with an incidence across the median plane of the optical system. The incidence per hour varies over the course of one and the same day, while the incidence with respect to the central plane of the collector experiences a change depending on the season for one and the same hour of the day. The second limitation mentioned above shows that in order to get a correct geometrical efficiency over the whole year it remains necessary to change the inclination of the collector, but that good results are obtained by taking only two positions, a middle position for the summer and a middle position for the winter.

In general, it is possible that the collector has a useful geometric efficiency in the following Areas retains:

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Focal length of the Fresnel lens from 150 to 250 mm

Focal length of the mirror from 35 to 45 mm

Distance lens-mirror from 130 to 150 mm

Distance mirror-tube from 25 to 35 mm.

Finally, another advantage of the collector according to the invention can be seen in the fact that he even in the absence of direct sunlight, namely alone with diffuse sunlight also works when the sky is overcast. This is easily explained by the fact that the efficiency of the collector is not depends on the azimuth of the sun; the mode of operation is correct for a change in altitude of about 40 °, since the diffuse energy a certain anisotropy at maximum strength in the direction of a ring surrounding the sun.

Technologically simplifies the fact that under favorable conditions a collector can be fixed at half the concentration Able to use its rack support considerably; this increases operational reliability; the total price of the plant decreases noticeably.

The main application of the collector according to the invention is in the heating of a heat transfer fluid such as water, glycol water, Pressurized water, organic heat transfer fluids, For the purpose of distillation sea water to be heated, chemical products to be heated and gases such as air and finally water vapor.

The device is always of interest when it is necessary to bring gases or liquids to a certain temperature Heating above 60 ° C is also interesting for heating liquids at low temperatures of about 50 ° C, especially in the case where the energetic density of solar radiation is low, especially in areas

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of the globe where energy occurs significantly in diffuse form, for example certain Nordic countries or certain tropical regions.

Other use cases are always possible where one tries to Concentrate solar energy, for example in the case of photoelectric cells, where it is sufficient to put them in place and to be arranged at the location of the absorber pipe described above. To get the thermal energy in the range of for photoelectric cells To eliminate useless and harmful wavelengths, the glazing can be advantageous on the flat surface through a a layer that is highly reflective in the near infrared range.

Of course, the device can also be used as a preheating stage for other types of collectors for a zone strongly increased temperatures are provided and those for the thermodynamic production, for example of electricity, are necessary.

Ordinary collectors can also be arranged in series

assign to each other for the preheating of the

Collectors with half concentration feeding liquid be used.

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Claims (9)

PATENT CLAIMS Y / solar collector with at least one Fresnel lens with ger-u, i inigcr) focussing zone and a cylindrical concave .Spiegel of essentially the same width, which is arranged parallel behind this lens and the light received from the lens on at least one between lens and reflective Throws back the target located on the surface, characterized in that the mirror has an approximately parabolic shape in (straight) cross-section and that the width (1) of the mirror and the width (L) of the lens, the focal length (f) of the parabolic mirror and the focal length (F) of the lens at normal incidence, the distance (d) of the target from the mirror and the distance (D) of the lens from the mirror and finally the width (t) of the focal spot formed by the lens with the width (c) of the target are linked by the following relationships, in which c = 3a: 030023/0815 ORIGINAL 1NSPECT £ D Γ = 'la ± O, 5a I · " = 2a t 13.5a + d = 3 a + 0, D. ■ = 5 a 2. Solar collector according to claim 1, characterized in that 1 - L ~ 15a f = 4 a + 0.2a F = 19.5a + 1.5_a t ^ 2a d = 3.2a ^ 0.2a D = 13.5a + _ 0.5 a .. 3. Solar collector according to one of the preceding claims, characterized in that the Fresnel lens (10) is a prismatic lens. 4. Solar collector according to one of the preceding claims, characterized in that the Tips of the prisms (12) of the Fresnel lens are all in one and the same plane. 5. Solar collector according to claim 4, characterized in that the lens consists of a disc rolled flat glass (20). 6. Solar collector according to one of claims 1 to 5, characterized in that the against the Lens facing surface of the target (16) is covered by a glass ribbon, the width of which is approximately c. 7. Solar collector according to one of claims 1 to 5, characterized in that the target (16) from an absorber tube with heat transfer fluid in one Sheath is made of thin glass. 030023/0815 8. Solar collector according to one of the preceding claims, characterized on the one hand by a glazing (20), formed from the assignment of several l ^; Resnel lenses (10), which are connected along their longitudinal sides and on the other hand a disc with a reflecting surface (22) with several parallel cells (14), the synergies of which coincide with those of the lenses and through a heat absorber (24) made of several tubes (16) ) arranged in the concentration zone of each of the optical systems formed by a lens and associated mirror, these tubes being connected in series. 9. Solar collector according to one of the preceding claims, characterized in that he adjustable and lockable in several positions on a fixed frame by means of an axis (36) which is parallel to the length of each of the optical systems and in the east-west direction of the central position oriented towards the sun. 030023/0815