Solar radiation receiver having an entry window made of quartz glass and method for producing an entry window

Abstract

Common solar radiation receivers are equipped with a chamber for transmission of an operating gas which is directed along to an absorber for solar radiation for thermal absorption. The absorber has a dome-shaped entry window made of quartz glass, wherein the inner side facing the absorber assumes a nominal interior temperature Ti of at least 950 C. during proper use, preferably at least 1000 C., whereas the outer side facing away from the absorber is exposed to the environment and subject to risk of devitrification. The invention relates to modifying the known solar radiation receiver so that a high absorber temperature can be set and thus a high efficiency of the solar thermal heating is enabled, without increasing the risk of devitrification in the region of the outer side of the entry window.

Claims

1. A solar radiation receiver comprising: a chamber configured to pass a working gas to an absorber for solar radiation so as to cause thermal absorption; the absorber being supported in the chamber and a dome-shaped window of quartz glass transmitting solar radiation has a convexly curved inside surface that faces the absorber, and an outside that faces away from the absorber and is exposed during operation of the receiver to an ambient temperature Tu, wherein the window has a wall thickness d such that, when the convexly curved inside surface has a nominal internal temperature Ti of at least 950 C. during operation of the receiver, the outside of the window has a temperature Ta that is less than 850 C. and at least 150 C. less than the nominal internal temperature Ti, and wherein the window has a region wherein the internal temperature Ti is a maximum internal temperature Ti of the window during operation of the receiver, and the wall thickness d in the region of said maximum internal temperature Ti is at least 7 mm; wherein the wall thickness d has a maximum in a dome center of the window; and wherein the wall thickness d varies from a minimum thickness to a maximum thickness, and said maximum thickness is greater than the minimum thickness by at least 20%.

2. The solar radiation receiver according to claim 1, wherein the wall thickness d is configured such that, when during operation of the receiver the internal temperature Ti is at least 950 C., the temperature Ta of the outside is less than 800 C.

3. The solar radiation receiver according to claim 1, wherein the wall thickness d is at least 10 mm in the region of the maximum internal temperature Ti.

4. The solar radiation receiver according to claim 1, wherein the wall thickness d in the region of the maximum internal temperature Ti is configured such that, when the internal temperature Ti is at least 950 C. during operation of the receiver, the wall thickness in said region conforms to the following dimensioning rule:
d>(TiTa)/(TaTu)(1), where: =a heat conduction coefficient of transparent quartz glass; =a heat transfer coefficient quartz glass/air; and Tu=25 C.

5. The solar radiation receiver according to claim 1, wherein the quartz glass of the window has a mean hydroxyl group content of less than 100 ppm by wt.

6. The solar radiation receiver according to claim 1, wherein the wall thickness d is configured such that, when during operation of the receiver the internal temperature Ti is at least 950 C., the outside has a temperature Ta of less than 750 C.

7. The solar radiation receiver according to claim 1, wherein the wall thickness d is at least 20 mm in the region of the maximum internal temperature Ti.

8. The solar radiation receiver according to claim 1, wherein the wall thickness varies from a minimum thickness to a maximum thickness, and said maximum thickness is greater than the minimum thickness by at least 50%.

9. The solar radiation receiver according to claim 1, wherein the quartz glass of the window has a mean hydroxyl group content of less than 30 ppm by wt.

Description

EMBODIMENT

(1) The invention will now be explained in more detail with reference to embodiments and a drawing. FIGS. 1 to 4 schematically illustrate method steps in performing the method according to the invention for producing a paraboloid-shaped window of quartz glass for a solar radiation receiver according to the invention. In detail:

(2) FIG. 1 shows the formation of a SiO.sub.2 grain layer in a melt mold;

(3) FIG. 2 shows the densification of the SiO.sub.2 grain layer by means of plasma;

(4) FIG. 3 shows a densified preform for the window after demolding and after grinding the outside;

(5) FIG. 4 shows the generation of a transparent wall of the preform by heating by means of a burner;

(6) FIG. 5 shows the window obtained according to the method; and

(7) FIG. 6 shows a diagram for determining the necessary minimum wall thickness of the window in response to the nominal absorber temperature.

(8) FIG. 7 shows a diagram showing a solar-radiation receiver according to the invention.

(9) FIG. 1 shows a melt mold 1 of graphite having a maximum inner diameter of 100 cm, which is positioned with an outer flange on a carrier 2 which is rotatable about a central axis 3. The space 4 between melt mold 1 and carrier 2 can be evacuated. The melt mold wall has a multitude of passages 5 through which a vacuum applied to the outside of the melt mold 1 can act on the interior 7.

(10) In a first method step, crystalline granules of natural high-purity quartz powder are filled into the melt mold 1 rotating about its longitudinal axis 3. The quartz powder has a multimodal particle size distribution with a main maximum of the particle sizes in the range of 50-120 m, the mean particle size being about 85 m, and with a secondary maximum of the particle size distribution at a particle size of about 1 m. Under the action of a centrifugal force and by means of a template, a rotation-symmetrical paraboloid-shaped layer 6 of mechanically compacted quartz sand is formed on the inner wall of the melt mold 1. The mean layer thickness of the layer 6 is 18 mm.

(11) In a second method step, the air contained in the grain layer 6 is enriched with a helium-containing process gas. To this end the air within the melt mold 1 is sucked off via the gas-permeable grain layer 6 to the outside, and a mixture of helium and 20% oxygen is simultaneously introduced into the interior 7 of the melt mold 1. The open upper side of the melt mold 1 is here partly covered with a heat shield 11 (see FIG. 2) while leaving a vent gap 12. After a period of about 10 minutes the enrichment with the helium-containing process gas is terminated.

(12) In a further method step, which is schematically shown in FIG. 2, the SiO.sub.2 grain layer 6 is densified zone by zone. To this end, after completion of the gas enrichment process, electrodes 8; 9 are introduced into the interior 7, and an electric arc, which is marked in FIG. 2 by the plasma zone 10 as a gray-shaded region, is ignited between the electrodes 8; 9 in the melt mold atmosphere consisting of helium and oxygen. In this process a constant and regulated flow of the He/O.sub.2 mixture of 300 l/min is further supplied to the interior 7, so that within the interior 7 a stable gas flow is formed between the gas inlet (not shown) in the heat shield 11 and the vent gap 12.

(13) Densification of the grain layer 6 is carried out in a two-stage process. In the first stage the electrodes 8; 9 are moved into a central position of the interior 7 and acted upon with a power of about 270 kW (200 V, 1350 A). The heat thereby generated in the interior 7 in combination with the process gas (80He/20O.sub.2) is enough for sintering the sinter-active particles of the grain layer 6, so that a thin, but dense sealing layer is formed over the whole inside thereof, the sealing layer separating non-molten portions of the grain layer 6 from the atmosphere in the melt mold interior 7.

(14) As soon as the sealing layer has been formed, the second vitrification step will begin. Due to the continued pumping action via the vacuum device, a negative pressure of about 200 mbar (absolute) is generated in the still unvitrified portion of the grain layer 6. The electrodes 8; 9 are now acted upon with a power of 600 kW (300 V, 2000 A) and moved into the vicinity of the inner wall and lowered downwards. It is thereby ensured that the inner portions of the grain layer 6 are reached with the plasma zone 10 and also with the process gas (80He/20O.sub.2). In this process a melt front is migrating from the inside to the outside, so that a portion of transparent, low-bubble quartz glass is obtained on the inside of the grain layer 6, as is outlined by way of the gray-shaded surface area 13. The thin and non-transparent sealing layer which covers the transparent inner portion 13 is at least partly removed in the further course of the process by the action of the plasma 10 and, if necessary, fully eliminated at the end of the production process by way of sandblasting.

(15) As soon as the transparent, vitrified inner portion 13 has reached a thickness of about 11 mm and before the melt front reaches the inner wall of the melt mold 1, the densification process starting from the inside of the SiO.sub.2 grain layer will be terminated. The adjacent portion 14 of the original grain layer 6, which is positioned further to the outside, is here densified into a porous quartz glass, whereas the outermost layer 15 of the original grain layer 6 remains undensified.

(16) After removal from the melt mold 1 the dome-shaped blank 16 of the window, which is schematically shown in FIG. 3, is thus obtained. In the blank 16, the inner portion is formed by a smooth, vitreous and low-bubble inner layer 14 with a thickness of 11 mm of quartz glass, which is firmly bonded to an outer portion 14 of bubble-containing quartz glass to which partly still undensified granules 15 are adhering. The adhering granules 15 are removed by sandblasting. The bubble-containing outer portion 14 is fully ground off, resulting in a dome-shaped preform 17 (see FIG. 4) of quartz glass with a uniform wall thickness of about 11 mm with an inner portion 13 of transparent quartz glass.

(17) As schematically shown in FIG. 4, the outer surface of the preform 17 which is still rough despite the grinding process is subsequently fire-polished. An oxyhydrogen burner 18 is guided along the outer wall of the preform 17 which is rotating about its longitudinal axis 20 (as outlined by the directional arrow 19), whereby the outer wall is locally heated to high temperatures of more than 2000 C. The rough outer surface is here fused without the transparent portion 13 of the preform being softened and significantly deformed in this process.

(18) As an alternative to this fire polishing process, the rough outer surface is smoothed by mechanical polishing and honing. The outer wall of the preform 17 is here processed by means of a honing machine, the degree of polish being continuously refined by exchanging the honing stone retainers. The final treatment is carried out with a #800 honing stone with a removal of about 60 m.

(19) After vitrification one obtains the dome-shaped window 2 of fully transparent quartz glass, as schematically shown in FIG. 5; it has a wall 22 with a uniform wall thickness of about 11 mm. The quartz glass of the window 21 has a mean hydroxyl group content of less than 50 ppm by wt. and an internal transmission of more than 95% (based on a layer thickness of 1 mm) in the wavelength range of 300-2400 nm. In the installed state the inside 23 is facing the absorber of the solar radiation receiver according to the invention and the outside 24 is exposed to the environment. The dome center of the window 21 or the apex of the dome is designated by reference numeral 25.

(20) In the diagram of FIG. 6, in the case of an absorber temperature Ti of 1000 C. the temperature Ta in [ C.] on the outside 24 of the window 21 is plotted on the y-axis against the wall thickness d in [mm].

(21) As a consequence, a maximum temperature of about 870 C. is obtained in the case of an window 21 having a wall thickness of 6 mm that is approximately within the range of the wall thicknesses that have so far been standard. In the case of a window wall thickness of 11 mm, as in the above embodiment, a temperature gradient of about 220 C. is formed over the window wall, so that the temperature on the outside 24 is only 770 C. This difference of about 100 C. in comparison with the standard wall thickness may be decisive for the long-term stability of the window.

(22) Inversely, a particularly thick-walled window 21 with a wall thickness of 11 mm (and more) at a given maximum temperature of e.g. 850 C. on the outside 24 permits process temperatures on the window inside 23 that are hotter by about 100 C., which considerably improves the efficiency of the conversion into electrical energy.

(23) FIG. 7 shows the dome-shaped window 2 installed in a chamber with an absorber in a solar-radiation receiver of known configuration.