AU2015392197A1

AU2015392197A1 - Mirror for concentrating sunlight for a solar power installation, method for operating a solar power installation and solar power installation

Info

Publication number: AU2015392197A1
Application number: AU2015392197A
Authority: AU
Inventors: Jürgen KLEINWÄCHTER
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-04-23
Filing date: 2015-08-04
Publication date: 2017-12-14
Anticipated expiration: 2035-08-04
Also published as: CN107810371B; DE112015006473A5; CN107810371A; DE102015009859A1; WO2016169537A1; AU2015392197B2

Abstract

The invention relates to a mirror for concentrating sunlight for a solar power installation, a method for operating a solar power installation and a solar power installation. Parabolic mirrors are known in the prior art. An important application of parabolic mirrors is the concentration of sunlight for the utilisation of solar energy. By means of concentrating using large parabolic mirrors, high temperature can be reached in the focal points thereof. The energy made available in this way can be used to melt metals or to produce steam. Small technical applications, such as solar cookers, also often use parabolic mirrors for concentrating solar energy. The invention proposes various advantageous mechanical and geometric approaches.

Description

Translation from German REFLECTOR FOR THE CONCENTRATION OF SUNLIGHT FOR A SOLAR POWER PLANT, METHOD FOR OPERATING A SOLAR POWER PLANT,

AND SOLAR POWER PLANT

The invention concerns a reflector for the concentration of sunlight for a solar power plant, a method for operating a solar power plant, as well as a solar power plant.

Parabolic reflectors are known from the prior art. One important application of parabolic reflectors is the concentration of sunlight for the utilisation of solar energy. High temperatures can be achieved in the focal point of large parabolic reflectors through concentration. The energy thus made available can be used to melt metals or generate steam. Small-scale applications, such as the solar cooker, often utilise parabolic reflectors to concentrate the energy of the sun . (Source: https ://de.wikipedia.org/wiki/parbolspiegel, retrieved on 13th July 2015.)

It is the object of the invention to provide the prior art with an improvement or an alternative.

According to a first aspect of the present invention this object is met by a reflector for the concentration of sunlight for a solar power plant, comprising a plurality of strip-like segments to concentrate incoming sunlight from a reflecting surface to a focus, in which the segments have a tangential extension, ie extend tangentially.

In this context, the following should be noted: A reflector of the kind relevant to this description concentrates incoming parallel light rays, that is, primarily sunlight, to a focus.

Reflectors of the kind described here have generally large dimensions, for example a diameter of more than one metre, quite often a diameter of even more than two or three metres.

Due to the size and for simplified production, such reflectors are usually arranged in strip-like segments. The strip-like segments are connected to each other and thus form the reflective surface.

According to the first aspect of the invention presented here, the segments have a tangential extension.

To put it simply, this means that the strips have the shape of a sector from an imaginary body with rotation symmetry, in which the strips are removed along the circumference of said imaginary body. A rotational paraboloid is the ideal reflector for concentrating sunlight. A rotational paraboloid (or paraboloid of rotation) is rotationally symmetrical about a central axis. A strip has an extension along a circumference about said axis, which is longer than the extension of the strip parallel to the axis.

In the prior art it is well known to assemble strip-like segments in the direction of the axis of the solid generated by rotation (rotational solid). This is because this kind of segmentation of the solid of rotation has the effect that all strips, and thus all segments, are identical in shape, which at first glance makes the production process simpler.

In contrast, the inventor recognised that the additional expense when using strips which extend in tangential direction is manageable. In particular deviations from the ideal shape of a rotational paraboloid are acceptable, as the losses caused by this are at the limits of the measurable range.

The reflector can still be constructed with a good approximation to the ideal using identically-shaped segments about the rotation axis, even at different levels .

However, preferred is an embodiment in which the reflector is comprised of segments with varying shapes. A particularly preferred embodiment provides that the reflector comprises identically-shaped, laterally adjacent segments at one level with respect to the rotation axis, but has different-shaped segments across the reflector height.

Preferably, narrower-shaped segments are provided at the exit end of the reflector, or the paraboloid, or its approximation, than at the vertex end. The less the segments extend in axial direction the smaller are the errors if the contour of the individual segment does not exactly correspond to a paraboloid of rotation but deviates from it.

One particular embodiment of the inventor provides that the longitudinally extending boundary edges, and thus the boundary edges that can be projected on the axis, of each segment are not geometrically shaped as parabolic sections but simply as circular arc sections. These can be produced and maintained much more cost-effectively. However, the further away the contour of the reflector is from the vertex, the more significant it becomes to minimise the errors caused by the intentionally "wrong" geometry. This can be achieved by using segments that are shorter in axial direction.

It is of course possible to assemble a parabolic reflector with the proposed tangentially extending segments .

However, based upon the prior art the inventor considers it smarter if a segment has edges that deviate from the form of a rotational paraboloid, and has primarily edges that are circular arc section-shaped.

The shape of the paraboloid of rotation is expensive to manufacture, which has a negative impact on costs. In contrast, it is significantly more cost-effective and also possible in technically lesser-developed regions to produce edges that are shaped as circular arc sections. The inventor recognised that the deviations from the shape of a paraboloid of rotation are less than can be justified by the added expenditure to produce a true paraboloid of rotation.

Thus at least one segment will have edges that deviate from the shape of the paraboloid of rotation.

Please note that within the scope of the present patent application generally the indefinite articles and number references such as "one", "two" etc are alxvays understood to mean "at least", such as "at least one , "at least two ..." etc, provided that in the context it is not specifically mentioned or implicitly clear or apparent to those skilled in the art that the meaning can only be or should be "exactly one ...", "exactly two ..." etc.

Compared to a full solid of rotation, the proposed reflector is provided with openings, in particular of at least 50% of the surface of the body of rotation.

When concentrating sunlight for a solar power plant, usually only a small portion of the overall surface is required, particularly in instances where the reflector has an intelligent tracking system.

According to a second aspect of the present invention, the object is met by providing a reflector for the concentration of sunlight for a solar power plant, comprising a plurality of segments to form a surface that reflects incoming sunlight to a focal point, in which the reflector can additionally correspond in particular to the first presented aspect of the invention, in which the reflector is characterised in that the segments comprise a shape-providing, pneumatic over or under pressure compared to ambient pressure.

With a design of this kind it is possible to shape the segments through the applied over or under pressure. This means that the segments have to be air-tight at least on the reflecting surface, or they have to seal at least against another kind of fluid, be that a gas or a liquid. The shape can be adjusted through introducing or discharging the fluid. The design may, for example, be such that the surface at a certain bandwidth around an ideal internal pressure takes on only a very small deformation compared to the ideal shape, thus not reducing the effectiveness of the solar power plant significantly. This also contributes to the fact that the invention can be utilised economically in regions that are less technically developed.

One particularly simple application provides for the inflating of the segments with air or the withdrawal of air. No special gas or liquid needs to be used, and the segments can be very light.

The segments can thus be easily collapsed, whether that is for transportation, maintenance or dismantling purposes. Multiple segments may be connected to each other via a fluid line. This design allows that multiple segments, in particular all segments, of a reflector can be shaped simply by introducing or discharging air or any other fluid. A segment may comprise a transparent film and a reflective film, where the transparent film and the reflective film are joined air-tight, in particular welded together, to form a pouch.

This design allows for the transparent film to be arranged such that it preferably faces the incoming radiation of the sun so that the sun rays pass through the transparent film and onto the reflective film. The reflective surface is thus located inside the pouch and therefore inside the, for example, inflated, cushion-like pouch when the reflector is in operation so that the reflective film is protected against dust contamination, for example.

If a segment is provided with a support frame, the mechanical loads can be taken up by the support frame, and it is thus possible to utilise very light designs for the reflective surface, such as films. A segment may comprise an inflatable tensioning element, in particular a tube, especially with varying internal pressure compartments along a circumference.

This way the tensioning element can be inflated, for example with air or any other fluid, be that a gas or a liquid; the foil-like, reflective surface is tensioned via the tensioning element and thus assumes the exact orientation to which the reflector is designed.

According to a third aspect of the present invention the object is met by a reflector for the concentration of sunlight for a solar power plant, comprising a plurality of segments for the formation of a surface that reflects incoming sunlight to a focal point, in particular a reflector according to one or two of the above-named aspects, in which the mirror is characterised in that the segments are shaped as reflective cushions comprising a fluoropolymer film.

Prototype tests by the inventor have shown that fluoropolymer films are ideal for the cushion-like reflectors .

Particularly suitable is ethylene tetrafluoroethylene (ETFE).

Moreover, thicknesses for the transparent film between 50 ym and 200 ym have proven to be ideal, in particular between approximately 100 ym and 150 ym.

Reflective foils with a layer of aluminium have proven to be ideal for the reflective surface, in particular with a sputtered aluminium reflector.

According to a fourth aspect of the present invention the object is met by a reflector for the concentration of sunlight for a solar power plant, comprising a plurality of segments for the formation of a surface that reflects incoming sunlight to a focal point, in particular a reflector according to one of the three previous aspects, in which the mirror is characterised in that the segments are provided with a reflective film, in which a mechanically reinforcing lattice structure is disposed on the reflective film, preferably on its non-reflective rear side.

Reinforcing "lattice structure" means that there are strip-like or seam-like raised sections in the thickness of the film that may be connected to each other.

The preferred lattice structure is diamond-shaped.

According to a fifth aspect of the present invention the object is met by a method for operating a solar power plant with a reflector for the concentration of sunlight for the solar power plant, comprising a plurality of segments for the formation of a surface that reflects incoming sunlight to a focal point, in particular for the operation of a solar power plant with a reflector according to one of the previously discussed aspects of the invention with respect to the reflector, in which the method is characterised in that the segments are deflated or inflated with gas to reduce the concentration effect in an emergency mode.

According to a sixth aspect of the present invention the object is met by a method for operating a solar power plant with a reflector for the concentration of sunlight for the solar power plant, comprising a plurality of segments for the formation of a surface that reflects incoming sunlight to a focal point, in particular for the operation of a solar power plant with a reflector according to one of the first four discussed aspects of the invention and/or according to a method according to the fifth aspect of the invention, in which the method is characterised in that the segments are subjected to fluctuating air pressure and thus are made to vibrate in order to clean their surfaces.

The amplitude that the segments assume at the surface due to the fluctuating air pressure is not especially significant. Rather, the vibration can be utilised to shake off snow or dust, for example.

According to a seventh aspect of the present invention the object is met by a solar power plant with a reflector for the concentration of sunlight, in which the reflector is mounted on a framework structure-like reflector support and reflects incoming sunlight to a focus, and where the reflector support is fitted with a preferably motorised daily tracking system, in which the daily tracking system is designed to rotate the reflector support around a swivel axis and thus follow the changing angle of the incoming sun rays, where the solar power station is characterised in that the swivel axis is aligned with the polar star if the solar power station is installed in the northern hemisphere, and the daily tracking system is set, if motorised, to rotate the reflector around the swivel axis with an angular velocity of 15°/min, but retain the focus and a receiver located at the focus in the same position.

In addition a seasonal tracking system is preferably provided, which is set to tilt the reflector around a tilt axis by at least 15°, preferably at least 20°, in particular approximately 23.5°, in which the tilt axis extends horizontally through the centre of the turntable. A solar power plant of this kind is of particular advantage if the reflector support is utilised for the tracking mechanics, and the reflector support carries a reflector that is as light-weight as possible, for example a cushion-like, inflatable reflector. Under consideration are mainly reflectors according to the previously described first four aspects of the invention, and/or reflectors in which the methods according to the fifth or the sixth aspect of the invention are applied.

According to an eighth aspect of the present invention the object is met by a solar power plant with a reflector for the concentration of sunlight, in which the reflector is mounted on a framework structure-like reflector support and reflects incoming sunlight to a focus, and where the reflector support is fitted with a preferably motorised daily tracking system, in which the daily tracking system is designed to rotate the reflector support around a swivel axis and thus follow the changing angle of the incoming sun rays, where the solar power station is characterised in that a control system is provided that comprises a focus sensor, a controller and a shaping motor, in which the controller has a data connection with the focus sensor and an operational connection with the shaping motor, in which the controller is set, in operation, to hold the focus of the concentrated sunlight at a target value through the shaping of at least one segment of the reflector.

It should be noted especially that the "target value" may also have a tolerance range, where the tolerance range is preferably set in the controller.

The "shaping motor" must be designed such that it is able to control at least one segment, preferably all segments, of the reflector, all together or individually, to the designated focus. Thus it is conceivable that air slowly escapes from a cushion-like reflector segment. For example, the shaping motor would be able, via a pump, to pump more air into the segment and/or to adjust the segment on its edges through one or more axes, in an ideal case through all six spatial degrees of freedom.

Adjacent segments may be attached to each other along their edges, or they may be individually freely adjustable, that is, arranged adjacent to each other but not attached.

The invention will now be explained in more detail by way of further description of the background theory underlying the invention, as well as non-limiting embodiments of the invention provided with reference to the drawings .

Shown are in:

Figure 1 a schematic representation of a paraboloid of rotation with a focus F in which all light rays that arrive vertical to the entry plane of the paraboloid concentrate,

Figure 2 a diagram that compares the intercept factor with that of the ideal paraboloid across the diameter of the receiver aperture at the equinox position of the reflector,

Figure 3 a three-dimensional graph of the intercept curve of a fixed-focus reflector in equinox position assembled from six circular drum sections,

Figure 4 a schematic cross-section of a transparent, flat film and a reflective, flat film,

Figure 5 a schematic perspective view of a section of reflective film with reinforcing lattice structure,

Figure 6 a schematic perspective view of a partial section of a segment,

Figure 7 a schematic representation of a crosssection through a reflector segment (already shown in Figure 4) with a mounting attachment,

Figure 8 a schematic representation in cross-section of a under-pressure reflector,

Figure 9 a schematic perspective view of a membrane paraboloid with six segments in light-weight construction,

Figure 10 a diagram of the achievable intercept factors with the reflector geometry depicted in Figure 9,

Figure 11 explaining the mechanical background, a schematic perspective view of an infinitesimally small surface element of a film that is shaped under a pressure p,

Figure 12 explaining the geometric background, a schematic perspective view of a paraboloid of rotation with designated surface elements, and

Figure 13 a schematic representation of the geometry of tracking the fixed-focus reflector described here .

The present embodiments describe the construction, the functionality as well as the major areas of application of an extremely light-weight, eccentric, fixed-focus, quasi-parabolic sunlight concentrator. The reflectors, bounded by special profiles, consist of arrangements of transparent and reflective polymer membranes whose surface shape is formed by air over or under pressure (vis a vis ambient pressure).

The essential, underlying ideas of the inventors are: a) Extremely low weight per unit area of the corresponding reflectors - thus lower energy requirement for their production ("grey energy") -quick energy-related amortisation. b) High surface quality (also minimal roughness) of the tensioned, reflective membrane. c) Parabola-like forming of the membrane within its elastic expansion range through targeted application of gas (air) over pressure or under-presure.

In his study "Concerning the tensional state of circular sheets with reducing bending stiffness" the Dutch physicist H. Hencky formulated in 1913 the equation for describing the equilibrium shape, which thin films that are circular in their periphery and are securely clamped take on when air pressure is applied:

(1).

Where wr represents the displacement of a membrane in z-axis .

When differentiated, this equation results in:

(2).

For a parabola the gradient of the surface as a function of the radius is characterised by a linear function. The second term in equation (2) is, however, a third order term, which shows that the "Hencky" membrane is steeper than a parabola at the periphery (similar to the spherical aberration of a spherical mirror). H. Kleinwachter calculated the tensional states (longitudinal and transverse tensions) in such a membrane deformed by air pressure and recognised that an originally flat membrane has to be systematically and anisotropically pre-tensioned so that it assumes an exact parabolic shape after a given application of pressure. An infinitesimally small surface element of a film is now observed that is deformed by the pressure p. The bending stresses in the film are assumed to be negligibly small; compare Figure 11.

In the state of equilibrium the equilibrium of forces between the force Fp caused by the pressure and the forces Fa and Fa caused by the retracting stresses can be established.

(3)

Based on diagram a) in Figure 11 the values can be inserted, where the stress components for small angles, which are opposed to the pressure, behave as shown in diagram b) of Figure 11.

(4)

Converting the equation (4) leads to

(5) .

For this it is assumed that the film deformed under pressure takes on the form of a paraboloid rotating around the z-axis with the focal point f.

(6)

Due to the rotational symmetry, and via the calculation of the principal curvatures, it is possible to determine both radii of curvature of the paraboloid p\ (in relation to the parallels) and pi (in meridian direction) in dependence on only the value x (compare Figure 12).

The quotient of both radii of curvature to each other results then in

(9).

The expansion of the film in one direction in the surface element analysed is the result of two components. The first component is the expansion, which is caused by the tension acting in this direction. The second component is caused by the transverse contraction, which results from the tension acting in the orthogonal direction.

(10) E designates here the elastic modulus of the film material and v its Poisson's ratio, which describes the transverse contraction behaviour in the material when stretched. The equation from (10), solved for σι and σι, results in

(11) and (12).

The equations (11) and (12) are now applied to (5), resulting in

Summing up the above consideration, it can be said that a flat film to which pressure is applied takes on the form of a paraboloid if the film is tensioned anisotropically as described earlier.

This consideration resulted in the concept of the fixed-focus concentrator. The arrangement of a number n of identical film segment reflectors (typically trapezeshaped) rotate around the polar axis at a continuous 15°/h and reflect the sunlight via a small aperture (focal plane of the reflector) into a cavity receiver. The seasonal adjustment (± 23.5°) takes place via a second axis, which extends through the aperture plane (see Figure 13).

Different prototypes with aperture areas ranging from 2 m2 to 20 m2 were built. Average sunlight concentrations of significantly more than 1000 Suns (average concentration of c > 1000) were achieved. This opened up the possibility to utilise cavity receivers and to effectively achieve process temperatures of up to approximately 2000°C. The main advantage of the fixed-focus concept over the classical paraboloid reflectors (where the receiver must follow the sun tracking movement) lies in the mechanical decoupling of heavy, stationary receivers with and without storage effect from the moving, light-weight optical system.

The prototypes were used to demonstrate various fields of application : • Solar cooking around the clock by using steel or sand as storage medium; • Operation of a thermo-chemical, reversible Mg - Mgfk storage for the base-load operation of a Stirling engine; • Thermo-catalytic receiver for splitting H2S into H2 and sulphur; • Coupling of the light into stationary light conductors; • Metallurgy and ceramics.

However, the conversion of the eccentric paraboloid segments with anisotropic pre-tensioning into an efficient, economically viable mass production has so far not been achieved for the following reasons: 1) Complex and time-consuming mechanical pre-tensioning method; 2) Creeping processes in the film, necessity for readjustment ; 3) Since the pre-tensioning must only occur in the elastic range of the film extension, low pretensioning forces cause sensitivity to wind forces in the reflector elements, causing deformation of the reflecting membrane.

The inventors of the present invention have made it their objective to develop a light-weight parabolic reflector that retains and improves the advantages of the fixed-focus reflector described in the previous chapter (fixed, stationary focus, low weight, formation of reflector through gas/air pressure) but avoids its inherent weaknesses (complex anisotropic pre-tensioning, deterioration of the image due to flowing plastic, timeconsuming and expensive production).

This required two significant development steps and insights that go beyond the prior art.

Firstly, the idea was conceived to form the fixed-focus reflector not from long strips in meridian direction, but from long strips in rotational direction of the paraboloid.

Fig. 1 reflects this fact. Shown is the original paraboloid of rotation (1) with the focus F in which all light rays that arrive perpendicular to the entry plane are concentrated. The reference (la) systematically depicts three fixed-focus segments arranged in meridian direction, as they are known from the described prior art.

The lateral profiles of these segments (2a), which extend in meridian direction, must naturally follow the parabola shape of the original paraboloid and thus form parabola sections. In contrast, the short upper and lower profiles (2b) that delimit the segments (la) form circle segments. In order for them to deform parabolically under pressure, such known membrane reflectors must, as described, be pre-tensioned anisotropically in a targeted manner due to the constantly changing curvature in meridian direction.

If, however, the segments are, according to an aspect of the invention, formed in rotational direction, as schematically indicated by the three segments (lb), the short sides (3b) will also form a good approximation of a circular arc (since they extend only a short distance in the meridian direction). The long sides of the delimiting profiles (3a) form precise circular arcs in any case. Since the boundaries are forming circular arcs with constant radii of curvature per se, the anisotropic pretensioning of the reflector membrane is no longer necessary. The membrane, which is cut according to the surface of a truncated cone, only needs to be evenly pretensioned after attachment to the frame.

After application of controlled gas/air pressure, the individual segments (lb) form circular drum segments that are arranged one above the other, and the focal points of which combine in F. If the individual elements are sufficiently narrow, the arrangement according to an aspect of the invention of an eccentric paraboloid constitutes a very good approximation of the corresponding section of the ideal paraboloid, as seen in Fig.2.

Fig. 2 depicts comparatively the intercept factor (relative value proportional to radiation power) of the arrangement according to an aspect of the invention and the ideal paraboloid across the diameter of the receiver aperture in equinox position of the reflector. The concentration figures of the ideal fixed-focus paraboloid are only insignificantly better. In the graphical representation of the intensity distribution a deformation of the focal point is hardly recognisable. Due to the error in shape the focal point extends a little further in z-direction than in y-direction. The ideal paraboloid concentration could be further approximated through better sectioning of the reflector surface (smaller segments at the top than at the bottom) and an optimisation of the alignment.

Fig. 3 shows the intercept curve for a fixed-focus reflector in equinox position constructed from six circular drum sections according to an aspect of the invention .

The fact that, as described, the segments need only be pre-tensioned homogenously in rotational direction leads to the second innovative aspect of the invention compared to the described prior art: The homogenous pre-tensioning of the films.

This is clearly depicted in Fig. 4. Designated with (4) is a transparent, flat film, and (5) is a reflective, flat film. Films (4) and (5) are joined air-tight at their edge (6).

The pouch formed by (4) and (5) is made as a fitted truncated cone surface, which is later pulled over the frame structure (2 x 3a + 2 x 3b). An inflatable, elastic tube extends along the outer periphery of the frame or, alternatively, a non-elastic tire (7) that lies flat in its base state.

If a pressure pi is applied to the inside of the tube it will expand and apply a defined tension to the film. The original pre-tensioning of the film is not of great significance since the tube compensates for this as it expands and maintains a defined tension on the film. Temperature changes in the film are also compensated for by the tube in the same way.

(14).

This fact becomes clear when analysing the equilibrium of the tensional forces of the films and the opposing force on the inner frame (14). The tensional force of the film is gf, the thickness of the foil is cIf and the height of the inner frame is tiR. The shape-providing air pressure between the films (4) and (5) is p2, where pi»p2.

The tube (7) is held in position by the auxiliary profile (8) .

It can basically extend as a single tube around the entire frame structure (with a fixed channel in the corners 3a - 3b to prevent kinking) or it may be made from four individually inflatable, linear sections (2 x along 3a, 2 x along 3b).

The pressurised tube applies a constant pressure to its inner wall. Now it is possible to apply tension to the membranes also anisotropically. The tensional anisotropy can be controlled via the frame height Hr. Said anisotropy is also maintained during changes in film temperature. According to an aspect of the invention, the anisotropy may alternatively be also achieved in that the tube along the membrane edge is comprised of n number of partial sections at varying internal pressures instead of providing variations in the thickness of the edge profile .

The curvature radii of the edge welding seams (6) follow the curvature radii of the shape-providing profile (3a). The films (4) and (5) are preferably made from highly transparent and light-resistant fluoropolymer films which, due to their high melting points, are quite difficult to weld with a defined pressure using classic resistance heating methods. Therefore two other methods are preferably used that solve this problem elegantly: Welding using ultrasound or laser. Both methods enable a precise formation of the required contour of the weld seam (6).

The choice of materials for the reflective elements according to Fig. 4 is a neat option of the present invention. To satisfy the main criteria of low weight, high precision and long life, the following material combinations are preferred according to the current level of understanding of the inventors:

Material of the reflector cushion

Fluoropolymer films, in particular ETFE at thicknesses between 100 ym and 150 ym; sunlight transmission of the transparent film (4) h 95%. Life: > 30 years. Dirt-repellent. Hail-proof as pneumatic cushion. A sputtered aluminium reflector is preferably applied to the reflective film: this allows also ultraviolet sunlight to be concentrated in the focus since the films are highly transparent also to the natural UV spectrum (300 -400 nm) . Thus the reflector technology according to an aspect of the invention is also very well suited to a combination with photochemical and photo-catalytic receivers (which usually prefer a fixed placement).

Material of the reflector frame

For weight reasons, aluminium, if a metal is chosen. Particularly well suited are also non-metallic fibre compound materials.

Material of the pre-tensioning tube

Preferred are ETFE tubes due to their long life under the effect of light and their low friction coefficient - thus are easy to move laterally and vertically, which makes it easier to pre-tension the reflector membrane without creases .

The pre-tensioning tube (6) typically compensates, over a broad control range, pressure changes in the pneumatic reflector that are due to temperature changes in the environment and also due to temperature-related elastic properties of the films (4) and (5) . Should there be any flowing effect within the film, it can also be corrected that way.

However, to exclude any flowing altogether, particularly the reflecting film (5) can be made according to an aspect of the invention as a flexible, lattice-reinforced composite structure. Fig. 5 depicts schematically such a compound structure. (5) depicts a section of the reflective film, (9) depicts the lattice structure visible on its underside, and (9a) shows a typical diamond-shaped pattern of said flexible lattice structure, which allows good pre-tensioning longitudinally as well as in transverse direction. Such a film compound structure can typically be produced efficiently in the following manner, in particular according to the current state of fluoro-film technology: A thin lattice made from fibres of high tensile strength is laid out flat and then covered with a gel-like layer of "liquid fluoro film" in such a way that none of the undulations of the mesh come through. The joining of the fluoro side of the film (5) with the liquid foil surface is achieved through a light, full-surface pressure, and the liquid film is brought into the solid state through evaporating the solvent.

Besides avoiding the flow effect when using this type of film, the pre-applied pressure of the reflector cushion can be chosen so high that even the effect of strong wind will not significantly affect the optical precision of the elements. According to an aspect of the invention, the pre-tensioning tube (6) can also be used to achieve a further important function: When using concentrating solar paraboloids with high energy density in the focus, it can become necessary to "switch off" the energy supply through the radiation at short notice. In principle this could be achieved by quickly moving the reflector out of the sun-collecting position, or through folding out a protective screen into the radiation path. The first option necessitates an elaborate "rapid move mode" in the reflector tracking system, and the second method is faced with providing problematic screens with high heat loads. In the instance of the present invention the reflector geometry can be immediately "neutralised" through rapidly deflating the pre-tensioning tube.

If fluctuating pressure is applied to the tubes (6), a targeted vibrating cushion surface can be achieved, through which, according to an aspect of the invention, dust, dirt and snow can be shaken off (supported by the low surface adhesion of the fluoropolymer membrane).

The low weight of the light-weight construction of the membrane segments (1-2 kg/m2) is achieved by a design similar to that of a model aeroplane wing.

Fig. 6 depicts a partial section of such a segment. Besides the already described longitudinal and lateral profiles (3a, 3b), the auxiliary profile (8), the upper transparent film (4) and the lower reflective film (5), as well as the drum-shaped film cushion (lb), there is shown the transverse support (3c) . When inflating the cushion (lb) and the tube (6), which is not shown here, said transverse support prevents the frame from being unduly deformed through the transverse contraction force on said frame.

The segment shown schematically in Fig. 6 features, for the reasons described above, high optical quality despite the extremely light-weight construction. Due to the consciously chosen small dimensions of the profile frame (weight), it is, however, relatively sensitive to torsion in longitudinal direction. According to an aspect of the invention, said torsion sensitivity is turned into a system advantage. Since the individual reflector segments are mounted in a light-weight, torsion-resistant framework, which acts as reflector support, in an overall configuration, the ability to adjust the segments on installation into the reflector support is utilised.

In Fig. 7 the already explained cross-section through a reflector element of Fig. 4 is complemented by a mounting member (8a), which is connected through a length-adjustable brace (8b) to the reflector support framework structure (10) . According to an aspect of the invention multiple points of the reflector segment frame are connected to the framework structure in this manner. By visually observing the focal plane it is possible to fine-adjust individual segments this way.

The reflector segment described so far acts as pressure reflector since pneumatic pressure is applied between the upper, transparent film and the lower, reflecting film. This has the advantage that the reflector is protected from direct weather conditions, however, due to the rays having to pass twice through film (4) a reflection loss of approximately 10% occurs. An optical aluminium layer has a reflection capability of about 90%, which means that the effective optical efficiency is approximately 80%.

It is apparent from Fig. 8 that the pressure reflector described in Fig. 4 can in principle also be used as a under-pressure reflector through adaptation of the height of the profile (3a), which achieves a 90% optical efficiency. The height for profile (3a) must be chosen such that the reflecting film (5) and the transparent film (4) do not come in contact if in the space between (4) and (5) a focal length-dependent under-pressure is applied.

Fig. 9 depicts schematically, according to an aspect of the invention, the construction of an eccentric, lightweight membrane paraboloid with six segments.

The six reflectors are fastened in the manner discussed to the reflector support that takes the shape of a framework structure. The rotation axis of the parallactically mounted reflector extends through the centre of the turntable (11) and points (in the northern hemisphere) to the polar star. The angular velocity of the solar tracking system is thus a constant 15°/min. Due to the high light concentration, which is projected onto the focal plane in a relatively small field of solid angle, the light is fed through a pupil with the diameter of the focal spot into a highly effective cavity receiver (13) . The seasonal tracking (12) of the reflector as a function of the height of the sun (± 23.5°) (elevation) is achieved via a second rotation axis that extends horizontally through the centre of the turntable.

Due to the light-weight construction of reflector and reflector support, the eccentric torsional moments that occur when changing the reflector position, as well as the adjustment of the elevation is possible without an elaborate mechanical construction. A surface quality of approximately 3 mrad is achievable with pneumatically formed concentration reflectors. As can be seen in Fig. 9b, the reflector geometry depicted in Fig. 9 can achieve intercept factors of close to 100%.

The main aim of the invention is to utilise the great potential of the sun particularly for decentralised use in villages and residential areas of the south. High-performance solar optics which, due to the described characteristics of the invention, can be applied in form of cost-effective, light-weight and easy to install and maintain kits. They are able to make a significant contribution to the local autonomy, quality of life and value creation. A broad spectrum of applications - starting with solar cooking around the clock, to water treatment in concentrated, natural UV light, to the operation of simple Stirling engines for energy, electric power and cooling is thus made possible.

Claims

Claims

1. Reflector for the concentration of sunlight for a solar power plant, comprising a plurality of strip-like segments for the purpose of forming a surface that reflects incoming sunlight to a focus, characterised in that the segments extend tangentially.
2. Reflector according to claim 1, characterised in that it has differently-shaped segments.
3. Reflector according to claim 2, characterised in that narrower-shaped segments are provided at the exit end of the parabolic reflector than at the vertex end.
4. Reflector according to any one of the previous claims, characterised in that the reflector is shaped as a parabolic reflector.
5. Reflector according to one of claim 1 to 3, characterised in that a segment has edges that deviate from the form of a paraboloid of rotation, and in particular has edges of circular arc section shape.
6. Reflector according to any one of the previous claims, characterised in that, compared to a full solid of rotation, the reflector body is provided with openings, in particular over at least 50% of the surface of the solid of rotation.
7. Reflector for the concentration of sunlight for a solar power plant, comprising a plurality of segments for the purpose of forming a surface that reflects incoming sunlight to a focus, in particular a reflector according to any one of the previous claims, characterised in that the segments exhibit a shape-giving over-pressure or under-pressure compared to ambient pressure.
8. Reflector according to claim 7, characterised in that a segment has a transparent film and a reflecting film, wherein the transparent film and the reflecting film are joined, in particular welded, air-tight to each other to form a pouch.
9. Reflector according to claim 7 or 8, characterised in that a segment is provided with a support frame.
10. Reflector according to any one of the claims 7 to 9, characterised in that a segment is provided with an inflatable tensioning element, in particular a tube, especially having different internal pressure compartments along its circumference.
11. Reflector for the concentration of sunlight for a solar power plant, comprising a plurality of segments for the purpose of forming a surface that reflects incoming sunlight to a focus, in particular a reflector according to any one of the previous claims, characterised in that the segments are made in the form of reflective cushions using a fluoropolymer film.
12. Reflector according to claim 11, characterised in that a reflector cushion comprises ethylene tetrafluoroethylene (ETFE).
13. Reflector according to claim 11 or 12, characterised in that a transparent film is used with a thickness of between 50 ym and 200 ym, in particular between 100 ym and 150 ym.
14. Reflector according to any one of claims 11 to 13, characterised in that it comprises a reflector film with an aluminium layer, in particular with a sputtered aluminium reflector.
15. Reflector for the concentration of sunlight for a solar power plant, comprising a plurality of segments for the purpose of forming a surface that reflects incoming sunlight to a focus, in particular a reflectors according to any one of the previous claims, characterised in that the segments comprise a reflecting film, wherein a nonreflecting rear side of the reflecting film is provided with a mechanically reinforcing lattice structure.
16. Reflector according to claim 15, characterised in that the lattice structure is diamond-shaped.
17. Method for the operation of a solar power plant comprising a reflector for the concentration of sunlight for a solar power plant, comprising a plurality of segments for the purpose of forming a surface that reflects incoming sunlight to a focus, the method applied in particular in the operation of a solar power plant with a reflector according to any one of the previous claims, characterised in that in an emergency mode, the segments can be deflated or inflated with gas to reduce the concentration effect.
18. Method for the operation of a solar power plant comprising a reflector for the concentration of sunlight for a solar power plant, comprising a plurality of segments for the purpose of forming a surface that reflects incoming sunlight to a focus, the method applied in particular in the operation of a solar power plant with a reflector according to any one of the previous claims, characterised in that the segments are brought to vibration by means of fluctuating air pressure to clean its surface.
19. Solar power plant comprising a reflector for the concentration of sunlight, in which the reflector is fastened to a framework-like reflector support structure and incoming sunlight is reflected to a focus, wherein the reflector support structure is preferably fitted with a motorised daily tracking system, and where the daily tracking system is designed to rotate the reflector support structure about an axis of rotation for tracking the changing direction of incoming sunlight, characterised in that the axis of rotation is oriented to the polar star in the northern hemisphere when the solar power plant is installed, wherein the daily tracking system is set so that the reflector, if motorised, is swivelled at an angular velocity of 15°/min about the axis of rotation, but wherein the focus and a receiver located in the focus, remain stationary.
20. Solar power plant according to claim 20 [sic], characterised in that a seasonal tracking system is provided, which is set to tilt the reflector over at least 15°, preferably over at least 20°, in particular over approximately 23.5°, about a tilt axis, wherein the tilt axis extends horizontally through the centre of the turntable .
21. Solar power plant comprising a reflector for the concentration of sunlight, in which the reflector is fastened to a framework -like reflector support structure and incoming sunlight is reflected to a focus, wherein the reflector support structure is preferably fitted with a motorised daily tracking system, and wherein the daily tracking system is designed to rotate the reflector support structure about an axis of rotation for tracking the changing direction of incoming sunlight, characterised in that a control system is provided that comprises a focus sensor, a controller and a shaping motor, wherein the controller has a data connection with the focus sensor and an operational connection with the shaping motor, wherein the controller is set, in operation, to hold the focus of the concentrated sunlight at a target value through the shaping of at least one segment of the reflector .