AU2015392197B2

AU2015392197B2 - Mirror for concentrating sunlight for a solar power installation, method for operating a solar power installation and solar power installation

Info

Publication number: AU2015392197B2
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: 2021-07-29
Anticipated expiration: 2035-08-04
Also published as: DE102015009859A1; CN107810371A; WO2016169537A1; DE112015006473A5; AU2015392197A1; CN107810371B

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 always 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 pm and 200 pm have proven to be ideal, in particular between approximately 100 pm and 150 pm.

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 150, 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.

According to a further aspect there is disclosed a

reflector for the concentration of sunlight for a solar

power plant, the reflector comprising a plurality of

strip-like segments for the purpose of forming a surface

that reflects incoming sunlight to a focus,

wherein the segments extend tangentially and each

segment has edges that have a circular arc section shape.

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 cross section 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:

wr rr 2 gr4 (1. = 1-0,9 (- - 0,1 (1) wo

Where Wr represents the displacement of a membrane in z axis.

When differentiated, this equation results in:

d wr 0,9 0,1 3(2). = -22r-4 _r3 dr wo 2 a a4

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 F,, and F,, caused by the retracting stresses can be established.

F = F, + F, (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.

p-p 1 da 1 p 2 da2 = da 1 -s-p2 da 2 +o da 2 s-p 1 da1 1 2 (4)

Converting the equation (4) leads to

U1 +2 P P1 P2 S (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.

1 z(x) = X2 4f (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 pi (in relation to the parallels) and p2 (in meridian direction) in dependence on only the value x (compare Figure 12).

1 (() p1=2f 1+Pt (7)

(8) 1+ (x )21 P2=2f 2f

The quotient of both radii of curvature to each other

results then in

P2 x 2 -=1 + pi (2f) (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.

1 V 1 V El - ub -- 2 - bzw. E2 2 1 (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 ai and a2,

results in

E(Ej + vE 2 )

1_ 2 (11) and

E(E2 + vE 1 ) (12).

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

El -- V-- +E2(l 1) (P1 P2 P2 P1 sE(1)

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 trapeze shaped) 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 (± 23.50) takes place via a

second axis, which extends through the aperture plane (see Figure 13).

Different prototypes with aperture areas ranging from 2 m 2 to 20 m 2 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 20000C. 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 - MgH2 storage for the base-load operation of a Stirling engine;

• Thermo-catalytic receiver for splitting H2 S into H 2 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 re

adjustment;

3) Since the pre-tensioning must only occur in the

elastic range of the film extension, low pre

tensioning 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, time

consuming 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 pre

tensioning 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 pre

tensioned 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.

(P1 - P2) ~FhR 2 -dF (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 aF, the thickness of the foil is dF and the height of

the inner frame is hR. The shape-providing air pressure

between the films (4) and (5)is p2, where p1>>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 pm and 150 pm; sunlight transmission of the

transparent film (4) > 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/m 2 ) 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, light

weight 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 (± 23.50) (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

1. Reflector for the concentration of sunlight for a solar power plant, the reflector comprising a plurality of strip-like segments for the purpose of forming a surface that reflects incoming sunlight to a focus, wherein the segments extend tangentially and each segment has edges that have a circular arc section shape.

2. Reflector according to claim 1, having differently shaped segments.

3. Reflector according to claim 2, wherein narrower shaped segments are provided at an exit end of the reflector than at a vertex end.

4. Reflector according to any one of the previous claims, wherein the reflector is shaped as a parabolic reflector.

5. Reflector according to any one of the previous claims, wherein, compared to a full solid of rotation, the reflector is provided with openings across at least 50% of the surface of the solid of rotation.