AU7410981A - Reflector arrangement - Google Patents

Description

REFLECTOR ARRANGEMENT

The present invention relates to a reflector arrangement, especially for use with solar collectors, having a sheet of reflecting material which is stretched to radiation-concentrating shape. The invention has for its object to make it possible at low cost, to build reflector surfaces for concentrating solar collectors, such that there is obtained a high concentration of radiation towards a heat absorber which, consequently, can also be built at low cost and in com pact form. A solar collector operating with concentrated radiation has a high degree of efficiency with high operating temperatures and low thermal inertia, whereby also short sunny spells can be utilized.

The requirements placed upon reflecting surfaces for concentrating solar collectors are high precision as regards design, indifference to wind loads, high reflectance, low tendency towards corrosion and soiling by rain and leaves etc. As a result of these requirements, prior art types of open reflector surfaces are expensive, especially if one considers their working life.

The reflector arrangement according to the present invention is characterized in that the sheet of reflecting material is disposed in an airtight covering of wholly or partly transparent plastic film material and defines an upper chamber and at least one lower chamber, and that a fan is adapted to maintain different air pressures in said upper and lower chambers in order to keep the covering stretched about the sheet and to stabilize the shape of said sheet. A reflector arrangement of this type can be produced from very inexpensive materials and with simple construction technique. When heavily soiled or when aged, the covering is readily exchanged.

The invention will be described below with reference to the accompanying drawings which illustrate embodiments of the invention. Fig. 1 illustrates in cross-section a converging reflector surface. Fig. 2 is a top view of said reflector surface, Fig. 3 is a cross-section of a first embodiment of a solar collector, Fig. 4 is a vertical section of a second embodiment of a solar collector, and Fig. 5 is a longitudinal section of the solar collector according to Fig. 4.

The material of the converging reflector surface 2 according to the invention consists of a thin and tight sheet 3 with a reflecting top layer 31. Preferably, the material used for the sheet is a thin polyester film coated with aluminium in an evaporation process. Such a material has a reflection factor of 0.95.

The geometry of the reflector surface 2 is formed by means of an edge frame 5 having parallel long sides 51 and curved short sides 52 between which wires 6 parallel to the long sides 51 are stretched. The reflector sheet 3 is secured to the edge frame 5 and supported against the wires 6. The reflector surface 2 will thus be divided into a number of elongate rectangular subelements 201, 202, 203 ..., each having a slightly concave converging surface because of the sagging of the sheet 3 between the wires 6.

The different subelements 201, 202 ... are generally oriented such that incident parallel bundles of rays 901, 902 ... are reflected or concentrated in a common focal surface 10. Due to the fact that each subelement is in itself converging, the focal surface 10 can be made considerably smaller than the surface of a subelement. By dividing the reflector surface 1 in a larger number of subelements, it is possible, in the manner described, to produce a high radiation concentration.

To stabilize the sheet 3 in its above-mentioned shape, one proceeds, according to the invention, in the following manner. The edge frame 5 is joined along its outer edges 51, 52 to a portion of a transparent covering 1 which is arched across the reflector surface, such that a chamber 7 is formed, and to a portion of said covering 1 which is positioned beneath the reflector surface 2 and also is arched, such that a chamber 8 is formed. A fan maintains a slight excess pressure with in the chamber 7 relative to the chamber 8, whereby the sheet 3 is stretched.

The covering 1 preferably is made of transparent plastic film which is stabilized in relation to its surroundings by means of an inner excess air pressure which is slightly higher in the chamber 7 than in the chamber 8.

According to the invention, these pressure ratios are established in the following manner by means of a fan 11. The pressure side of the fan 11 is connected to the chamber 7, a number of exhaust openings 12 are provided in the sheet 3, and a throttled return passageway 13 is connected between the chamber 8 and the suc tion side of the fan 11. The restriction within the pas sageway 13 makes it possible to control a circulating air flow which, due to the flow resistances within the openings 12 and the passageway 13, respectively, provides for the desired pressures. The ratio of recirculated air to fresh air is controlled by means of a damper 16 at the suction side of the fan 11. It may be necessary to dry out condensate within the covering by increasing the air throughflow.

The covering 1 and the reflector surface 2 are preferably so designed that the focal surface 10 falls within the covering so that a radiation absorber 15 can be mounted within said covering to protect it against soiling and heat dissipating air movements caused by draught.Because of the high radiation intensity obtained in the focal surface 10 an additional cover glass in front of the absorber surface can be dispensed with. The embodiment according to Fig. 3 is intended for high concentration of direct solar radiation and, consequently, high operating temperatures and large-scale heat production. A solar collector module of this design is erected on the ground and in dimensions of, for example, the order 3x100 m.

A reflector surface 2a and a covering la are built up in accordance with what has been described in connexion with Figs. 1 and 2, and comprises an edge frame 5a with parallel long sides 51a and curved short sides 52a between which wires 6a are stretched, a reflector sheet 3a secured within the frame 5a, a transparent film 4a supported by an inner excess pressure and having an upper and a lower part, an injection fan 11a, exhaust openings 12a, a return ^passageway 13a for air, and the requisite supporting brackets 16 for the frame 5a.

The arrangement is oriented with its longitudinal axis in an east-west direction and with the reflector surface 2a curved towards substantially the northern part of the sky. Solar radiation of southerly incidence will be converged towards a focal surface 10a. The reflector surface 2a and the upper boundary surface 4a of the covering la are so designed that, according tothe solstice, the focal surface 10a will move across the rear portion of the said boundary surface 4a. This surface is covered by an absorber 15a built up by parallel absorption elements, for example black water conducting pipes oriented in the longitudinal direction. The water flow is conducted to the pipes 151a which at the moment are in the focal surface 10a, whereas unillaminated pipes are blocked against water throughflcw. In this manner a high degree of efficiency is obtained in the system reflector-absorber, regardless of the solstice, and this is achieved with fixedly mounted construction elements.

The concentration degree in the focal or concentration surface 10a depends upon the division of the reflector surface 2a into subelements 201a, 202a...

For example, a division into ten elements makes it possible to obtain a radiation concentration of 8:1. Since the division merely depends upon the number of laid-in wires 6a, extremely high concentrations are obtainable at marginal extra cost.

A second embodiment of a concentrating solar col lector module is shown in Figs. 4 and 5. This model is primarily intended for smaller installations having a relatively low operating temperature, for example for the production of domestic hot water. The dimensions of such a module may be, for example, 1.5x4.0 m with a total absorber surface of about 5 m², and a solar collector installation is constructed with an optional number of modules.

The arrangement comprises an edge frame consisting of two rigid vertical end panels 52b and two longitudi nal horizontal bent sheet metal sections 51b. The end panels are of symmetrical heart shape with cylindrically curved lower edges and straight upper edges forming a V. Along the vertical plane of symmetry, the end panels are interconnected by means of a rectangular, planar absorber 15b. Stretched over the said edge frame is a covering 1b of polyethylene film which is secured in sections 51b and along the edges of the end panels 52b, respectively. A reflector surface 2b of a reflecting sheet 3b is stretched between the end panels 52b and is anchored to the longitudinal frame sections 51b by means of tape or glue and supports itself against the lower edge of the absorber 15b. The sheet 3b divides the space within said covering into an upper chamber 7b and two lower chambers 8b. The pressure side of a fan 11 is connected to the chamber 7b, exhaust openings are provided in the sheet 3b, and a throttled return passageway 13b connects the chamber 8b to the suction side of the fan 11. With the arrangement in the inflated state, the outer covering lb is stabilized against wind loads, and the reflector sheet 3b constitutes two cylindrically curved surfaces 201b and 202b.

The arrangement is oriented with its longitudinal axis substantially in an east-west direction. Solar radiation incident upon the arrangement strikes both the absorber surfaces facing north and those facing south, both directly and by reflection upon the reflector surfaces 201 and 202, and this occurs substantially independently of the angle of incidence. In this manner, the arrangement is capable of absorbing both diffuse and direct solar radiation, substantially independently of the sun's position. Although the arrangement according to the invention is especially suited for solar collectors, it may of course also be used for other purposes where a radiation concentration is desired. Moreover, the radiation concentrating shape need not be part-cylindrical; with a suitably formed supporting frame a, for example, spherical surface Is readily accomplished.

Claims (6)

1. A reflector arrangement, especially for use with solar collectors, having a sheet (3) of reflecting material which is stretched to radiation-concentration shape, c h a r a c t e r i z e d in that the sheet (3) is disposed in an airtight covering (1) of wholly or partly transparent plastic film material and defines an upper chamber (7) and at least one lower chamber (8), and that a fan (11) is adapted to maintain different air pressures in said upper and lower chambers in order to keep the covering (1) stretched about the sheet (3) and to stabilize the radiation-concentrating shape of said sheet.

2. An arrangement as claimed in claim 1, c h a r a ct e r i z e d in that, for the supply of air, the pressure side of the fan (11) is connected to the upper chamber (7) which is in communication with the lower chamber (8) through pressure reducing openings in the sheet (3), and that the lower chamber (8) is connected to the suction side of the fan (11) via a passageway (13).

3. An arrangement as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the reflector sheet (3) is connected to the covering (1) around its periphery by means of a rigid frame (5) .

4. An arrangement as claimed in claim 3 , c h a r a ct e r i z e d in that the sheet (3) is supported in the frame (5) by means of wires (6) strung in said frame.

5. An arrangement as claimed in claim 4, in which the sheet (3) forms at least one part-cylindrical surface, c h a r a c t e r i z e d in that the wires (6) are strung parallel to one another in the frame in such spaced apart relationship that the part-cylinder is longitudinally divided into narrow strips concentrating incident radiation to a small area (10) within the covering (1), in which area a radiation absorber (15a, 15b) is provided.

6. An arrangement according to claim 3, c h a r a c t e r i z e d in that the frame is formed by two rigid vertical end panels (52b) which are heart-shaped with lower edges in the form of circular arcs and two straight upper edges forming a V, and longitudinal horizontal sheet metal sections (51b) connected to said end panels (52b) at the intersection between said arcuate and straight edges; that a radiation absorber (15b) is in the form of a planar disk, the opposite ends of which are connected to the two end panels (52b); that the covering (lb) is sealingly connected with the outer periphery of the end panels (52b) and the sheet metal sections (51b); and that the sheet (3b) is connected to the end panels at a distance inwardly from the arcuate edges thereof and at the inner periphery of said sheet -metal sections (51b) and extend downwardly beneath the downwardly facing edge of the absorber (15b).