ABSORBENT FOR THE CONVERSION OF SOLAR RAYS IN THERMAL ENERGY
FIELD OF THE INVENTION The invention relates to an absorber for converting solar rays into thermal energy, in particular for use in a solar collector, which is circulated from one side to the other by means of heat transport. BACKGROUND OF THE INVENTION Solar collectors are used to convert solar energy into thermal energy and as such to make it usable. These essentially comprise an absorbent, which is made of a material with good thermal conductivity, such as, for example, copper or steel, and a pipe system through which a liquid or a gas transports the energy absorbed from the absorbent towards a site of application of the thermal energy obtained. To increase the operating temperature of a solar collector, in addition to the optical equipment such as, for example, heliostats or parabolic troughs, they can be used in order to focus the sun's rays on the absorbent. The absorbers usually have a black surface, which is obtained through the application of a black pigmented paint, in order to ensure a maximum absorptive capacity of solar energy. However, the
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disadvantage with this is that whenever the absorber is used in a solar collector with corresponding capacity, such as, for example, a solar thermal power plant, in which large-area optical systems are used to concentrate the incident sunlight for the absorbent in order to achieve high temperatures of the absorbent, this can lead to a destruction of the absorbent coating or paint job. In order that this coating or paint job and its coloration are not destroyed too quickly, the absorbers in turn are therefore kept in evacuated glass tubes in order to prevent access of oxygen, which does not However, on the one hand it increases the costs and on the other hand it requires a regular cleaning of the evacuated glass tubes, since otherwise they could be heated and with this destroyed, which, however, could necessitate the change of the corresponding plant. BRIEF DESCRIPTION OF THE INVENTION In contrast, an object of the invention is to provide an absorbent, subjected to side-by-side flow by a heat transport means, to convert the solar rays into thermal energy, in particular for use in a collector solar, which also allows a low-cost and maintenance-free application to a large extent, when
They use high capacity solar collectors. This object is achieved according to the invention since the absorber essentially comprises a dark non-porous ceramic material. The central concept of the invention is to use, instead of using metal pipes with a dark paint or coating, in particular black, in the use of non-porous ceramic pipes which inherently comprise a dark material, which on the one hand has the advantage that the absorbent does not need to be specially blackened, and on the other hand it eliminates the need to prevent oxygen access through encapsulation in vacuum tubes. By using the ceramic means there is also a possibility to allow absorbent temperatures much higher than 400 ° C, in particular up to 800 ° C and even higher, depending on the optical equipment used. According to the invention, it is proposed with this that the non-porous or waterproof ceramic is a non-oxide ceramic based on silicon carbide (SiC), in particular technical silicon carbide, which among other things has a high thermal conductivity and a Low thermal expansion, and can also be used at very high temperatures. The technical silicon carbide is dark (black to green) due to the impurities present, the degree of coloration decreases as the degree of
purity of silicon carbide. Above all, silicon carbide sintered without pressure (SSIC) and silicon carbide infiltrated with silicon bonded by reaction (SISIC) has proved that they are particularly suitable non-oxide type ceramics, based on silicon carbide, although the carbide of sintered liquid phase silicon (LPSIC), hot pressed silicon carbide (HPSiC) and hot isostatically pressed silicon carbide (HIPSIC) can also be used. The Sintered Sintered Silicon Carbide (SSIC) is produced from ground finer SIC powder, which is processed with sintering additives in the customary ceramic forming and sintering variants from 2000 to 2200 ° C under inert gas. SSIC is characterized by a high strength that remains virtually constant up to high temperatures of approximately 1600 ° C. This material also has a high resistance to thermal shock, high thermal conductivity, high resistance to abrasion and a hardness similar to that of diamond. In contrast, silicon carbide infiltrated with silicon bound by reaction (SISIC) is composed of approximately 85 to 94% SIC and consequently 15 to 6% silicon metal. In addition, the SISIC virtually does not
It has residual porosity. This is achieved since an article molded of silicon carbide and carbon is infiltrated with silicon metal. The reaction between the liquid silicon and the carbon leads to a SIC binding matrix, where the residual pore space is filled with silicon metal. The advantage of this production method is that, in contrast to powder sintering techniques, during the siliconization process, the components do not suffer any shrinkage. Therefore, extraordinarily large or long absorbers can be produced with precise dimensions. Although the range or range of use of the SISIC is limited to approximately 1380 ° C due to the melting point of the metal silicon, up to this temperature range SISIC has a high resistance and corrosion resistance combined with good resistance to thermal shock and the resistance to abrasion. In summary, silicon carbides are thus characterized by properties such as high hardness, resistance to corrosion even at high temperatures at high abrasion resistance, high resistance even at high temperatures, resistance to oxidation up to very high application temperatures. high, good resistance to thermal shock, low thermal expansion and very high thermal conductivity. In particular, the low thermal expansion is particularly advantageous if the absorbent is formed
in a tubular manner or, as it is shaped in one embodiment of the invention, it is composed of a plurality of tubular elements densely placed one on the other. Absorbents of this type are used in particular in solar power plant using collectors type gutter or parabolic trough comprising curved mirrors that condense the solar energy on an absorbent tube that runs in the focal line, whose tube is fixed with fasteners in the focal line of the collector. The lengths of such collectors and thus also the length of the absorbent tubes used can be between 20 and 150 meters, depending on the type, wherein the individual tubular elements to each other usually have a length of approximately 2 to 4 meters. In addition, the aforementioned properties of silicon carbide make it possible to do so to a large extent without the measures provided in the prior art to absorb the expansion of length, to support the weight and to prevent deformation at high temperatures of the absorbent materials used. In order to make possible a simple connection of a plurality of exemplified absorbers in a tubular manner to form a simple absorbent element, according to the invention, this can be provided for the connection of two tubular elements to be carried out by a
sliding joint. This type of connection allows a quick assembly, but has the disadvantage that it is sensitive to longitudinal forces, which, however, occur only to a slight degree through the use according to the invention of ceramics of the non-oxide type with a low thermal expansion. However, it is also conceivable to use flange-type seals or screw seals to connect two tubular elements. However, metal clamps can also be provided to be used to secure a sliding joint, which prevent longitudinal forces from occurring in the absorbent by loosening the sliding joints. Due to the properties of silicon carbide, for example silicon carbides infiltrated with silicon bonded by reaction (SISIC), which does not undergo any shrinkage during the production process, and the individual tube segments can be produced very precisely, which could even be made possible to connect the individual tubular elements to each other with positional precision and tightly without additional sealing. However, the sealing of the sliding joints can also be provided according to the injection, to be carried out by means of a silicon seal that is
adapted to the tubular shape of the tubular elements, or so that the seal of the sliding joint is carried out by means of a mastic or refractory adhesive. Liquid or gaseous heat transfer fluids, such as water, liquid sodium, isobutane, thermal oil or superheated steam, etc., can be used as a means of heat transport. If thermal oil is used as a means of heat transport, temperatures of up to 390 ° C can be reached, which are used in a heat exchanger to generate steam and then fed to a conventional steam turbine. The superheated steam, however, is used by direct steam generation, which does not need a heat exchanger, since the hot water vapor is generated directly in the absorber tubes and fed to a steam turbine, which makes possible temperatures above 500 ° C, when using parabolic trough collectors. If, in addition, the absorbent according to the invention is used with solar energy plants in which the solar radiation is concentrated on a central absorber with the help of hundreds to thousands of automatically positioned (heliostatic) mirrors, maximum temperatures of approximately 1300 are possible. ° C. According to the invention, it is also possible to provide the heat transport medium comprising
silicon oil, which is characterized by low volatility, low viscosity and temperature coefficients, fireproof character and high strength with respect to acids and alkalis, but also has high electrical resistance and low surface tension. In addition, silicon oil is neutral in terms of smell and taste and physiologically indifferent. Through the use of the absorbent according to the invention with a heat transport medium based on silicon oil, temperatures well above 400 ° C, in particular up to 800 ° C, sometimes even higher, can be reached. BRIEF DESCRIPTION OF THE FIGURES The exemplary embodiments of the invention are described in more detail below with reference to the figures. These show: Figure 1, a perspective representation of a parabolic trough collector containing the absorbent according to the invention; Figure 2, a cross section through the joint of two tubular absorbers connected to each other by a sliding joint; Figure 3, an enlarged view of the sliding joint shown in Figure 2; Figure 4, an enlarged view of a sliding joint according to another embodiment of the invention,
similar to Figure 3; and Figure 5, an enlarged view of a sliding joint according to still another embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a perspective representation of a parabolic trough converter 10. The parabolic trough converter 10 has an extended reflector 12 which as a rule is made of glass which is coated with silver and thus acts as a mirror. In the cross-section the receiver 12 has the shape of a parabola, and in the focal line (not shown) of the reflector 12 an absorbent 14 is located which comprises a plurality of individual absorbent tubular elements 16, in which absorbent circulates a means of heat transport, such as, for example, silicon oil, thermal oil or water vapor. The heat carrier and the structure of the absorbent tube 14 are not shown in Figure 1. The reflector 12 assembled from a plurality of reflector elements 13 has a support structure 18, which essentially comprises a plurality of carriers 20 coupled to each other. the base and a structural frame 22, which is used to support the individual reflector elements 13 in a deformation-free manner, and to carry support elements 24 with the
The absorbent tube 14 is always maintained in the focal line of the reflector 12. In order to make it possible for the reflector 12 to track the sun, additional rotary impellers (not shown) are provided which allow an oscillating movement of the structural frame 22 with with respect to the carriers 20 on the oscillating pivots 26, one of which is shown. On the opposite ends 28 and 30 of the absorbent 14, the absorbent is connected to a linear system 32 having an inlet 34, through which the heat transport medium is introduced to the absorbent 14, and an outlet 36 through which is introduced. from which the heat transport medium is discharged. Depending on the heat transport medium used, it is also possible to guide the medium of. heat transport either first to a heat excer (not shown) for steam generation, which is then fed to a conventional steam turbine, or if superheated steam is directly generated in the absorbent tubes, to feed it directly to a steam turbine without interposition of a heat excer. Figure 2 is a cross section through a connection of two absorbent tube elements 16, 16, which in the present embodiment are jointly part of the absorbent 14. Each member 16 of absorbent tube comprises
a ceramics of the non-oxide, non-porous / impermeable type based on silicon carbide, which is present in technical form and is dark. The silicon carbide used has a very high hardness, a very high resistance to corrosion even at high temperatures, high abrasion resistance, high resistance even at high temperatures, resistance to oxidation up to high application temperatures, good shock resistance thermal, low thermal expansion, very high thermal conductivity and good tribological properties. Preferably, silicon carbide sintered without pressure (SSIC) and silicon carbide infiltrated with silicon joined by reaction (SISIC) are used, which due to its production process makes it possible that the components do not suffer any shrinkage during the process of siliconization, through which extraordinary large components can be produced, with precise dimensions. Since the additional silicon carbide has only low thermal expansion, very long absorbers 14 with correspondingly large individual absorbent tube elements 16 can be used, without extensive consideration having to be given to the axial extension of the absorbent tube 14. No However, alternatively, silicon carbide sintered in liquid phase (LPSIC) or hot pressed silicon carbide (HPSIC) and silicon carbide
hot isostatically pressed (HIPSIC) can be used, which also belong to the group of waterproof or non-porous silicon carbides. The technical silicon carbide used has a dark color (light green / dark green, black, gray) due to the impurities present, depending on the degree of purity, so that it is not necessary to obscure the tubes in a special way, which means that it is not necessary or prevent the access of oxygen by encapsulation in the vacuum tubes in order to prevent a very rapid destruction of the coating of the absorbent or the coloration, as is necessary with the prior art. As shown in Figure 2, two elements of the absorbent tube 16, 16 are connected by a sliding joint, with which a tip end 38 of an absorbent tube element 16 is inserted into the bell 40 of a pipe element. adjacent absorbent 16, which allows a quick assembly. The direction of flow of the heat transport means is shown in Figure 2 by an arrow 42, and runs from the bell of a tubular element towards its tip end. In addition, the sliding joint can be additionally secured via retaining clamps or retaining screws (not shown) in order to prevent loosening or loosening of the sliding joint. Figure 3 shows an enlarged representation
of the sliding fit of Figure 2, which shows that the sliding joint is further sealed by a mounting adhesive or glass water adhesive 44 which does not contain organic solvents, is non-combustible and can be used at much longer temperature intervals high of 1000 ° C. Alternatively, however, other refractory adhesives or mastic agents that have temperature resistances of up to 1700 ° C or ceramic adhesive substances can be used, which have temperature resistance up to 1700 ° C, or ceramic adhesive substances are used, which are inserted in liquid form in the connection region of the two absorbent tube elements and then harden the connections of the absorbent tubular elements 16, reliably. Figure 4 shows an alternative embodiment of the invention in which two absorbent tubular elements 16, 16, as in Figure 3, are connected to each other via a sliding joint, in which a tip end 38 of an absorbent tubular element 16 is inserted into the bell of the other absorbent tubular element 16. In addition, however, with this embodiment a silicon seal 46 is provided in order to seal the sliding seal. In the embodiment shown in Figure 4, the tip end 38 of an absorbent tubular element 16 is provided with a phase 48, so as to facilitate, on the other hand, the
introduction of the tip end 38 into the bell 40 of the other absorbent tubular element 16, as well as to serve as a guide assistant for the silicon seal 46, so as not to attack it during the insertion of the tip end 38 within the bell 40. With the embodiment shown, the tip end 38 as well as the bell 40 has a respective notch or recess 50 or 52 to accommodate the silicon seal 46 (which in Figure 4 is shown as an O-ring) in order that in this way an additional fixation of the two absorbent tubular elements 16, 16 connected to each other is made possible. Figure 5 shows another embodiment of the invention in which two absorbent tube elements 16, 16 as in Figure 4 are connected to one another via a sliding joint, in which a tip end 38 of an absorbent tube element 16 is inserted into the bell 40 of the other absorbent tube member 16, wherein the sliding joint is sealed by means of a silicon seal 54. However, this embodiment differs from the embodiment shown in Figure 4 in that the end of tip 38 is not bevelled, as shown in Figure 4, but is exemplified with a tapered tip end 38 in the form of a bottle neck and a trumpet-shaped bell 40 exemplified in a correspondingly opposite manner. In addition, with this modality of the elastomeric seal of the
The bell (silicon seal) 54 is not inserted into a channel or recess, but lies flat between the tip end 38 and the bell 40, in order to seal the connection area. With the assembly of the elastomer bell seal 54, it is placed "dry" on the tip end 38 of the other absorbent tube element 16 (left), then covered on the outer side with a lubricant, likewise on the side of the bell 40 of the other absorbent tube element 16 (right). Now the absorbent tube element 16 (left) with the seal attached is placed over the bell 40 of the other absorbent tube element 16 (right) and together with the bell seal 54 pressed into the bell 40, where the end of tip 38 centers itself in the bell 40 through the conical shape of the bell seal 54. The bottleneck shape of the tip end 38 shown in Figure 5 is selected only by way of example and can be exemplified in a different way, depending on the bell seal used, for example, with a longer or shorter neck, with different thickness of material, etc. In addition, this type of sliding joint can be additionally secured via a retaining clamp or retaining screw in order to prevent a looseness or loosening of the sliding joint. However, in addition, it is possible to use, instead of
the bell 40 which is coupled to one end of a respective absorbent tube element 16, exemplifying these ends as tip ends and providing separate bells which are pushed over the two tubular elements to be connected. However, alternatively it is also possible to use other types of connection, such as, for example, flange connections or screw connections, depending on the material properties of the silicon carbide used in order to make pressures possible in this way. higher in the absorbent. It is noted that in relation to this date the best method known by the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.