WO2011033546A2

WO2011033546A2 - A reflecting- panel having a thin mirror and a support made of composite material smc (sheet moulding compound) for linear parabolic solar concentrators

Info

Publication number: WO2011033546A2
Application number: PCT/IT2010/000389
Authority: WO
Inventors: Mauro Vignolini; Adio Miliozzi; Daniele Nicolini; Arcangelo Benedetti; Giuseppe Cara; Aldo Nunzi
Original assignee: ENEA - Agenzia nazionale per le nuove tecnologie, l'energia e lo sviluppo economico sostenibile
Priority date: 2009-09-18
Filing date: 2010-09-10
Publication date: 2011-03-24
Also published as: ITRM20090476A1; EP2478308A2; WO2011033546A3; IT1398366B1

Abstract

A reflecting panel (1), comprising a supporting layer (2) with parabolic geometry and a thin glass (3) applied to the concave surface (4) of the support, wherein said support (2) is a stiff layer formed starting from a sheet-moulding-compound (SMC) composite material comprising randomly arranged glass fibres, impregnated with thermoplastic resins.

Description

A REFLECTING PANEL HAVING A THIN MIRROR AND A SUPPORT

MADE OF COMPOSITE MATERIAL SMC (SHEET MOULDING COMPOUND) FOR LINEAR PARABOLIC SOLAR CONCENTRATORS

Technical sector

The . present invention relates to the sector of concentrating solar-power plants for the conversion of solar power into thermal, chemical, or electric power.

In this type of plants, the reflecting part of the linear concentrating system is constituted by a series of panels that provide a cylindrical-parabolic reflecting surface, i.e., a surface the right section of which is a parabola, which has the function of concentrating the solar rays on a receiver tube, traversed by a thermovector fluid.

The receiver tube is thus responsible for conversion -^'of the solar power into thermal power, which is used in turn for producing electric power or for sustaining chemical processes.

The invention regards in greater detail a thin- mirror reflecting panel with linear parabolic geometry, of the type constituted by a stiff support, applied on which is, for example via an adhesive, a thin ^' glass. that constitutes the reflecting surface..

Panels of this type have been used in the production of concentrating solar-power plants and can be deemed a technical component that is important in exploitation of solar energy in the immediate future.

State of the art

In the current state of the art, cylindrical- parabolic solar concentrators are known made up of mirrors made- of thermally tempered curved glass having a thickness of 4-5 mm and a parabolic shape, which rest on a lattice structure constituted by hollow tubes arranged according to a lattice, which provides the stiffness necessary .for countering the deforming force due to the action of the wind.

In these known concentrators, the panel is then formed by a self-supporting glass and is rested directly upon the underlying structure.

Solar mirrors of this type present the characteristic of being difficult to handle and brittle so as to requires in some cases reinforcement with glass fibres for improving their mechanical characteristics .

These known panels are moreover relatively complex to assemble and present relatively high production costs.

Also known are parabolic reflecting panels, referred to as "thin-mirror panels", i.e., constituted by a mirror of .small thickness (approximately 0.85 mm) applied to a stiff support constituted by a sandwich with fibre-glass skins and honeycomb aluminium core.

An example of embodiment of thin-mirror panels is described in the patent application No. R 2001A000350.

As compared to self-supporting glass panels, this technical solution has proven able to offer a greater manageability and ease of installation, as well as a- greater safety in regard to possible yielding, of the glass, which remains in any case glued to the structural support.

In this case, a process for the production of sandwich mirrors, referred to as "vacuum-bag technique", is currently used.

According ^'to this technique, the fabric impregnated with resin and deposited on the corresponding mould is enclosed in a bag of polymeric material in which the vacuum is formed and maintained. The pressure exerted by the bag and the heat supplied - from outside enable polymerization of the material, which assumes the shape of the mould imposed.

Said process has, however, highlighted production costs that are still too high to enable a commercially rapid development.

In the international patent application No. W02008145551, a technical solution is moreover described for the production of thin-mirror panels with which it is proposed to use chemically tempered thin mirrors, coupled to supports that guide the shape thereof to obtain parabolic reflecting surfaces.

Unfortunately, even though the process of chemical tempering considerably increases the characteristics of strength and deformability of- the thin glass, it is in turn a technically complex and costly process.

Furthermore, thin-mirror panels of a known type presents limits in relation to the stiffness offered to deformations induced thermally or by the mechanical action of the wind and of the interaction with the . supporting structures.

A further limit of known panels is then constituted by the possibility of automating the entire production process.

Purpose of the invention Consequently, a first purpose of the present invention is to provide a thin-mirror panel that will be light and at the same time stiff in regard to thermal and mechanical stresses, reducing the possible points of initiation of cracks in the glass.

A further purpose is to provide a support for thin-mirror panels that will be compatible with the glass applied and workable using automated techniques.

Yet a further purpose of the invention is to propose a support for the glass that will ensure high dimensional stability.

Summary of the invention

The^' above purposes have been achieved with a thin- mirror reflecting panel according to at least one of the annexed claims.

In particular, according to the invention it has been devised to provide a reflecting panel comprising a mirror made of thin glass applied to a stiff supporting layer formed starting from a sheet-moulding-compound (SMC) material.

The advantages obtained basically consist in enabling a higher strength-to-weight ratio as compared to the supports already known thanks to the good specific characteristics (i.e., referred to the -density) of SMC materials.

In particular, the main specific advantages in the production of reflecting panels that derive from the use of SMC materials, are:

-. high performance-to-weight ratio;

- high mechanical and thermal resistance;

- high values of dielectric strength; - self-extinguishment and total absence of halogens;

- low specific weight;

- high dimensional stability;

- high resistance to the corrosive action of chemical and atmospheric agents; and - ^'

- long service life and no need for maintenance.

A further advantage consists in the simplicity of assembly and installation, and hence in the need for smaller workforce.

Yet a further advantage is represented by the fact that SMC materials are made up to a large extent of inert fillers and glass fibres, the availability of which does not substantially depend upon raw materials (for example, ones of petroliferous origin) , which would involve unstable costs and market conditions.

Yet a further advantage is represented by the fact that the composition of SMC materials can be readily modified in order to obtain specific desirable characteristics on the basis of the requirements.

Yet a further advantage lies in the fact that the support made of SMC can be obtained by hot forming in steel moulds, with multiple technical advantages in terms of cost and of encumbrance both in comparison with injection presses for thermoplastic materials and in comparison with injection presses for drawing of sheet metal, at the same time maintaining a high production capacity.

List of the drawings

The above and further advantages will be better understood by any person' skilled in the branch from the ensuing description and the annexed drawings, which are provided by way of non-limiting example and in which: Figure 1 shows in perspective view a reflecting panel^' according to the invention;

Figure 2 shows a cross section of the panel of Figure 1;

Figure 3 shows a detail of the points of rest of the panel of Figure 1;

Figure 4 shows the arrangement of two panels according to the invention to form a semi-parabola of the reflecting surface of a solar concentrator with a linear parabolic configuration; and

Figure 5 is a schematic illustration of a solar concentrator with a linear parabolic configuration provided with the panels according to the invention.

Detailed description of the invention

With reference to the attached drawings and in particular to Figures 1 and 2, a reflecting panel 1 is described, which comprises a supporting layer 2 with parabolic geometry, and a thin glass 3 applied to the concave surface 4 of the support, for example via a layer 18 of adhesive.

Preferably, the edges of the glass 3 can be ground in order to limit to the minimum the edge defects and the likelihood of. breaking of the glass. Furthermore, the strength of the thin glass 2, and hence its curving capacity for its application to the support, can be increased through . appropriate methods of thermal or chemical tempering, which are in themselves known.

According to the invention, the supporting layer 2 is constituted by a stiff layer formed starting from a sheet-moulding-compound (SMC) composite material and comprising 'randomly arranged glass fibres, pre- impregnated with thermoplastic resins.

Preferably, the SMC material comprises unsaturated polyester resins, mineral fillers, and glass fibres, in addition to other components, such as catalysts, thickening agents, thermoplastic additives, detaching agents, other minor components, and possible pigments.

The composition of the support 2 may in any case vary according to the applications .

By way of example,' an SMC with physical and mechanical properties compatible with typical applications _. in concentrating solar-power plants can have a weight percentage of glass fibres of- between 30 wt% and 50 wt%, of mineral fillers of between 5 wt% and 30 wt%, and of unsaturated polyester resin of between 20 wt% and 25 wt%.

Advantageously, it has been found that by varying the composition of the SMC material, with particular reference to the amount of glass fibre, fillers, and resin, significant variations can be obtained in. the chemico-physical characteristics of the finished product according to the various applications. In greater detail, it has been found that, by using a content of glass fibres comprised approximately between 40 wt% and 50 wt%, the dimensional stability of the panel is greater when subjected to the day/night thermal variations that the panel is bound to undergo during use.

The greater stability of the support 2 in turn improves the compatibility with the mirror made of thin glass since it brings the values of coefficient of thermal expansion of the two parts closer once they have been coupled together, thus limiting possible fractures or cracks of the glass, or deformations of the geometry of the panel.

In one example of embodiment, it has been verified that, given a coefficient of -thermal expansion of glass (a thin glass of a sodium-calcium type in a single plate with a nominal thickness of less than 1.5 mm) close to 8 x i0⁶/°C, the support 2 made of SMC with 50 wt% of fibre has shown a^' coefficient close to 12 x 10⁶/°C, ranging between 10 x 10⁶/°C and 15 x 10⁶/°C.

With reference to Figures 1 and 2, a distribution of . stiffening ribs 9 is described, devised in order to increase the stiffness of the support 2 when this is attached in a number of attachment points 8 to a supporting structure 17 of a solar-power plant, for example a concentrator of the type illustrated in Figure 5.

In the preferred example described, each panel 1 is provided with four attachment points 8, which are arranged on the convex surface 5 and from which the ribs 9 irradiate.

According to one aspect of . the invention, the support 3 is obtained by pressing by means of hot forming starting from one or more pre-impregnated sheets of composite . SMC material.

Advantageously, with this solution steel moulds can be used^' of dimensions similar to those of the panels 1 to be obtained, with a considerable reduction in encumbrance and costs as compared to the equipment necessary for the production of supports already known, at the same time maintaining a high productivity.

With reference to Figures 4 and 5, described in greater detail is the use of the panels according to the invention in a solar concentrator 14 with linear parabolic geometry.

According to the invention, the concentrator 14 comprises a distribution along a longitudinal axis X of pairs of panels 11, 12 arranged, respectively, in an internal position, i.e., adjacent to one another, and an external position in the two directions transverse with respect to the longitudinal axis X of ^~ the concentrator.

With this solution, each panel 11, 12 corresponds approximately to one quarter of the arc of a parabola that constitutes the right section of the reflecting surface of the concentrator, and each pair of panels 11, 12 defines a stretch in length of a semi-parabola of the reflecting surface itself..

In a non-limiting example of embodiment, the reflecting surface of the concentrator 14 has a transverse aperture L, measuring 5900 mm in length, and a focus F set at 1810 mm from the vertex V of the parabolic surface.

Furthermore, the reflecting surface does not originate in the axis X of the parabola, but is shifted transversely by 50 mm in so far as the band 15 under the line of the foci F is in the shade, of the receiver tube 16 (constituted, for example, by a tube traversed by a thermovector fluid for conversion of the solar energy into thermal energy) and is thus unproductive. With this solution, the development in the plane of the reflecting surface per unit length comes to correspond exactly to twice the surface of two panels.

In the preferred example described herein, the width of the panels has been chosen as 1600 mm, and the thickness of the panels, including the points 8 for anchorage to the supporting structure 17, may be comprised between 5- and 25 mm, but it is understood that different dimensions may be chosen according to the application.

Claims

1. A reflecting panel. (1), comprising a supporting layer (2) with parabolic geometry and a thin glass (3) applied to the concave surface (4) of the support, said panel being characterized in that said support (2) is a stiff layer formed starting from a sheet-moulding-compound (SMC) composite- material comprising randomly arranged glass fibres., impregnated with thermoplastic resins.

2. The panel according to Claim 1, wherein said support (3) is made up of glass fibres in a weight percentage of between 40 wt% and 50 wt% .

3. The panel according to Claim 1 or Claim 2, wherein said composite material has mineral fillers in a weight percentage of between 5 wt% and 10 wt%, and unsaturated polyester resin in a weight percentage of between 20 wt% and 30 wt%.

4. The panel according to any one of the preceding claims, wherein said support (2) comprises on the convex surface (5) a distribution of attachment points (8) and a plurality of stiffening ribs (9).

5. "The panel according to Claim 4., comprising at least four attachment points (8), from which said stiffening ribs (9) irradiate.

6. The panel according to any one of the preceding claims, wherein said thin glass (3) is of a sodium-calcium type in a single plate with a nominal thickness of less than 1.5 mm.

7. The panel according to any one of the preceding claims, wherein said thin glass (3) is glued to said support by means of an adhesive designed to guarantee adhesion of the thin mirror to the support made of SMC and to prevent, at the same time, transmission to the glass of any excessive stresses that could initiate fractures.

8. The panel, according to Claim 7, wherein said adhesive is selected between the categories of MS polymers and epoxy-resin-based films.

9. The panel according to any one of . the preceding claims, wherein said support (3) is obtained by pressing by means of hot forming starting from one or more pre-impregnated sheets of said SMC material.

10. A solar concentrator with linear parabolic geometry, comprising at least one panel according to any one of the preceding claims.

11. The concentrator according to Claim 9, comprising a distribution along a longitudinal axis (X) of pairs of panels that are internal (11) and panels that are external (12) with respect to said axis (X) , said panels (11, 12) each ¹ defining one quarter of a parabola of a linear portion of the reflecting surface designed to concentrate solar radiation (R) impinging upon a receiving concentrator device (16) set along the focus (F) of said surface.