The present invention refers to a two-stage thermal sun concentrator.

As known, the art proposes different types of sun devices that allow converting the sun irradiation into thermal energy by means of components aimed to capture the sun radiation. In general, the operation of such devices provides that the sun radiation, once having reached the capturing device, is absorbed by an absorbing device and transferred to a thermo-vector fluid, that can be water, air or a diathermal fluid.

The part of irradiation that directly reaches the ground is the direct radiation, while the remaining part is the diffused radiation. To this, finally, the reflected radiation or "albedo" must be added, that is the percentage of direct and diffused radiation that is reflected from the ground or the surrounding surfaces onto the affected surface: some of the sun devices are able to exploit only the direct radiation, while other ones allow using the three components (direct, diffused and reflected) of the radiation itself.

The sun devices can also be classified depending on the temperature of the thermo-vector fluid and on the concentration ratio Cr, defined as the ratio between the admission surface of the non- concentrated sun irradiation and the device absorption surface.

Currently, the sun devices are substantially- represented by solar plants, solar concentrators and panels.

In order to build solar plants, normally tower-type systems are used, whose major elements are: a mirror field, composed of a high number of reflecting surfaces that automatically follow the sun path and that concentrate the sunrays instant by instant towards a receiver; an energy receiver (punctual boiler) , placed on a tower located in a central position with respect to the mirror field; a system for converting thermal energy firstly into mechanical energy (steam turbine) and afterwards into electric energy (electric generator) ; an adjusting system arranged to keep the mirrors orthogonal to the direct radiation.

The operation of following the sun can be performed by a computer o by photosensitive elements that, instant by instant, measure the orientation error of the single mirror.

The concentrators instead are composed of a mirror or of optical lenses, that converge the sunrays towards the absorber in which the thermo- vector fluid flows. Since they exploit only the direct radiation, they need devices adapted to keep at any time the reflecting surface orthogonal to the sunrays direction.

Moreover, the concentrators are divided into "image"-type systems, which are more common, that reproduce the sun image onto the focal plane, and into "no image"-type systems, that randomly concentrate the sunrays onto the absorber.

The image-type concentrators can in turn be of a spot or linear type, if they converge the sunrays into the focal point or into an axis passing through the focus .

The main image concentrators of the spot type are the parabolic concentrators, characterized by a parabolic reflecting surface and by an absorber placed in the focal area.

Among them, the two main types are distinguished by sun following: the first type has its absorber fixed and integral with the reflector that instead is mobile and follows the sun; the second type instead has the fixed reflector and the mobile absorber, that is directed in the area in which the reflector converges the sun radiation.

The cylindrical-parabolic concentrators instead are image-type systems of a linear type: they are composed of a reflecting surface obtained by translating a parabola along an axis passing through its focus and orthogonal to the plane that contains it. In the focal area of the reflecting surface, the linear absorber is placed, generally composed of a copper or stainless steel piping within which the thermo-vector fluid flows. In order to reduce convection losses and to enable the greenhouse effect, the piping can be placed inside a glass tube.

As regards following the sun, the system can have the fixed absorber and the rotary parabola or have the absorber integral with the parabola in turn subjected to the rotary motion. Following the sun finally can be on an axis (and in such case the absorber will have to be oriented along the east- west direction) or on two axes.

Finally, the solar panels are composed of: an absorbing surface; a network of pipes in which the thermo-vector fluid flows; a transparent cover; an insulating coating; a containing structure that is the external envelope .

Plane solar panels use the three components of the sun radiation and exploit the green house effect. The transparent cover in fact is made with transparent materials to the incident sun radiation, but opaque to the re-irradiated infrared radiation. The thermal energy coming from the sun is therefore captured inside the panel and transferred to the thermo-vector fluid. In order to limit the heat losses towards the outside, the side and rear areas are then protected with insulating material .

Plane solar panels are suitable for low- temperature applications, differently from other sun devices that are more suitable for medium- and high-temperature applications. Plane solar panels are for this reason preferred for civil uses, also because they can be easily integrated into buildings; concentration-type collectors instead require their own supporting and handling structures .

This structure has a series of very valuable construction advantages. In fact, it is much simpler to build many plane mirrors instead of a big curved mirror. The support structure of such a device, being less exposed to wind-generated forces, is simpler and lighter that the one that uses a paraboloid. In addition, it is easier to handle the individual plane mirrors that reflect the sun energy on the secondary mirror. A further advantage is having a fixed received, simplifying the part related to plants.

Object of the present invention is solving the above prior art problems by providing a two-stage medium/high-temperature thermal sun concentrator that can be optimally placed to allow the incident sunrays to be reflected by a primary mirror onto a secondary mirror that conveys the sun radiation into an absorber.

Another object of the present invention is providing a two-stage thermal sun concentrator in which the whole sun radiation that strikes the primary mirror is addressed and concentrated inside the opening angle of the secondary mirror.

Moreover, an object of the present invention is providing a two-stage thermal sun concentrator adapted to shadow the most exposed surfaces of buildings, thereby contributing to their thermal regulation during the various seasons.

The above and other objects and advantages of the invention, as will appear from the following description, are obtained with a two-stage thermal sun concentrator as described in claim 1. Preferred embodiments and non-trivial variations of the present invention are the subject matter of the dependent claims .

It will be immediately obvious that numerous variations and modifications (for example related to shape, sizes, arrangements and parts with equivalent functionality) can be made to what is described, without departing from the scope of the invention as appears from the enclosed claims.

The present invention will be better described by some preferred embodiments thereof, provided as a non-limiting example, with reference to the enclosed drawings, in which: FIG. 1 shows a top perspective view of a preferred embodiment of the two-stage thermal sun concentrator according to the present invention;

FIG. 2 shows a bottom perspective view of the two-stage thermal sun concentrator of FIG. 1;

FIG. 3 shows a side sectional view of a preferred embodiment of the two-stage thermal sun concentrator according to the present invention;

FIG. 4 shows a side sectional view of a component of the two-stage thermal sun concentrator according to the present invention;

FIG. 5 shows a side sectional view of another component of the two-stage thermal sun concentrator according to the present invention;

FIG. 6 shows a side sectional view of the two- stage thermal sun concentrator according to the present invention in some operating modes thereof;

FIG. 7 shows a schematic view representing an operating principle of the two-stage thermal sun concentrator according to the present invention; and

FIG. 8 to 16 show some design modes of the two-stage thermal sun concentrator according to the present invention;

FIG. 17 shows a variation of the secondary mirror, namely of the component shown in FIG. 5;

FIG. 18 shows another preferred embodiment of the concentrator of the invention made of an involute section and a macrofocal parabola section;

FIG. 19 shows some of the technical solutions devided for connecting two extruded parts composing the concentrator of FIG. 18; and

Fig. 20 shows the workings performed in order to be able to insert the insulating cupels in the concentrator of Fig. 18.

With reference to Figures 1 to 7, it is possible to note that the two-stage thermal sun concentrator 1 according to the present invention comprises a first stage consisting in at least one primary mirror 3 and a second stage consisting in at least one secondary mirror 5, such primary mirror 3 being adapted to collect the incident sunrays . R]_ and to concentrate and reflect them R2 in such secondary mirror 5, such secondary mirror 5 being adapted to collect the rays R2 and to concentrate and reflect them R3 in at least one thermal absorption element 7.

With particular reference to FIG. 7, it is then possible to note that the operating principle of the two-stage thermal sun concentrator 1 according to the present invention is based on the concentration of the sun radiation obtained through the use, in cascade, of a reflecting surface 3a of the primary mirror 3 and of a reflecting surface 5a of the secondary mirror 5 designed according to the "no image" optics principles, by empirically proceeding using as basic references the methods of strings used for creating Compound Parabolic Concentrators (CPC) . In particular, with reference to Figures 8 to 12, in order to obtain the desired operation, the curvature of the reflecting surface 5a of the primary mirror 5 has been defined as follows .

In order to realize a geometric outline that is able to reflect all rays incident towards the thermal absorption element 7, the Applicant has chosen to use a spiral having a ratio A/B between the radius of the arcs that generate it (as shown, in particular, in FIG. 8) . Such spiral in fact has the optical feature of reflecting the incident rays orthogonal to the surface towards the centre of the curve that describes it: in this way, also through internal reflections, the incident rays always get to be reflected along the direction of the various focuses, and the positions of the four focuses (or centres of the arc describing the spiral) circumscribe the position within which the thermal absorption element 7 must be placed.

In particular, with reference to Figures 8 to 12, the process followed to empirically build the spiral comprises the following steps: a) drawing a circumference of rays A in the square 1 and having its centre in point a (see FIG. 9) ; b) generating a second arc of circumference with centre in b and rays B; c) drawing other squares, which are smaller and smaller, arranged as a spiral as shown in FIG. 10, stopping when generating the fourth square, the rays of the n-th arc being equal to the rays of the n-th-1 arc x A/B, obtaining the complete spiral as shown in FIG. 11.

In order to increase the entry surface for rays reflected by the reflecting surface 3a of the primary mirror 3e and to obtain a total smaller encumbrance of the concentrator 1 according to the present invention, it is possible to limit the first arc of circumference inscribed in square 1 by about 45° (as shown, for example, in FIG. 12) and reduce the arc inscribed in the last square.

In parallel, the curvature of the reflecting surface 3a of the primary mirror 3 has been made according to the principle called "edge-ray" that, as shown in particular in FIG. 13, provides that, if a ray at the end of the surface, incident according to the maximum acceptance angle, is transferred to the opening edge, this is enough to transfer all incident rays within the acceptance range of the output opening (Winston 2005) : this is equivalent to state that the beam of incident light on the surface is wholly reflected on the output opening.

In order to make the reflecting surface 3a of the primary mirror 3, therefore, an empirical method has then been used, that defines the particular surface that, for a given incidence angle of the sunrays, totally reflects them within the slit of the secondary mirror 5. In particular, the first part of the curvature of such reflecting surface 3a is determined by searching spot by spot which angle must have a linear mirror with infinitesimal sizes to reflect the incident ray onto the absorber surface; as shown in FIG. 14, the incident ray is traced that strikes the surface spot I and the vector is computed that is normal to the mirror in the spot I itself, so that the reflection laws are observed in order to make the reflected ray centre the spot A.

The above process is then repeated for every incident ray r till a spot is reached that is tangent to the surface of the thermal absorption element 7 : the definition of such spots and their inclination allows generating the outline of the reflecting surface 3a (FIG. 15) .

After having defined the first portion of curve, the same method is followed to define the remaining part, using as target the reflecting surface 5a of the secondary mirror 5 instead of the thermal absorption element 7 (FIG. 16) .

The particular curvature of the primary mirror 3 has then been computed to address and concentrate the whole sun radiation that strikes it within the opening angle of the secondary mirror 5.

The resulting "snail"-type curvature of the reflecting surface 5a of the secondary mirror 5 further concentrates the sun radiation received from the primary mirror 3 and reflects it uniformly on the whole surface of the thermal absorption element 7.

Obviously, the mirrors 3 and 5 are made of an adequate material to resist to exposure to atmospheric agents and ultraviolet UV rays.

The thermal absorption element 7 is preferably composed of an exchanger made of a metallic tube inside which at least one thermo-vector fluid flows. The exchanger is preferably wound onto wings to optimise the thermal exchange and is contained inside a vacuum glass tube equipped with at least one selective borosilicate coating.

In the preferred embodiment of the two-stage thermal sun concentrator 1 according to the present invention, shown in particular in Figures 1 to 5, it is possible to note that the primary mirror 3 is composed of at least one supporting portion of the reflecting surface 3a onto which the sunrays R]_ are incident, to be then concentrated and reflected R2 towards the secondary mirror 5, such supporting portion of the reflecting surface 3a being fitted to at least one concave portion 3b inside which the secondary mirror 5 is housed and fastened: in this way, obviously, the primary mirror 3 and the secondary mirror 5 are mutually moved during the orientation of the two-stage thermal sun concentrator 1 towards the sun radiation. Moreover, the thermal absorption element 7 is arranged inside the concave recess of the reflecting surface 5a of the secondary mirror 5.

The two-stage thermal sun concentrator 1 according to the present invention can further be equipped with a single-axis positioning system in order to keep an optimum orientation thereof with respect to the sun elevation in various seasons/day times: with particular reference to FIG. 6, it is possible to note that the two-stage thermal sun concentrator 1, and preferably the primary mirror 3, can be externally equipped with at least one fastening portion 9, preferably made as a bracket, adapted to allow the connection of at least one motoring means (not shown) , such as for example a linear actuator, of the above positioning system to allow the rotation of the concentrator 1 itself (as shown for example by arrow R-R in FIG. 7) around a rotation axis that is preferably horizontal and coaxial with at least one rotation shaft 11 and the integral positioning of the mirrors 3, 5 in their optimum position for capturing the sun radiation.

In particular, the positioning system is composed of at least one motoring means, such as a linear actuator, driven by electronic control means arranged for receiving positioning data from a centralized system. In particular, such control means automatically compute the positioning data upon setting information such as, for example, latitude, date, time, orientation and/or altitude to sea level and, through a suitable process or algorithm, they optimally orient the two-stage thermal sun concentrator 1 according to the present invention through the action of the motoring means to allow the incident sunrays to be reflected by the primary mirror onto the secondary mirror, that, in turn, conveys the sun radiation onto the thermal absorption element 7.

Obviously, information to the positioning system can be set manually or supplied automatically through the use of satellite positioning tools.

The positioning system further allows positioning the two-stage thermal sun concentrator 1 in a safety position (null radiation concentration and shadowed thermal absorption element 7) compared, for example, with reaching over-temperatures or in case heat immission is not required any more.

It is further advantageous to note that the two-stage thermal sun concentrator 1 also allows a favourable architectural integration on south- exposed facades of civil and industrial buildings. This in fact allows coupling the sun exploitation as energy resource to the need of shadowing the most exposed building surfaces, thereby contributing to their thermal regulation during the various seasons.

FIG. 17 shows another preferred embodiment of the secondary mirror 5' of the sun concentrator of the invention. In this Figure, the secondary mirror has been made in order to improve system performances: this part of the device is the one adapted to convey on the absorbing tube 7 all sun radiations reflected by the primary mirrors 3.

The need of having a reflecting secondary mirror 5' is due to the evaluation of the errors that are made in approximating the parabola through plane mirrors. Moreover, other errors to be compensated are the non-null error made in computing the sun position and the positioning error of the primary mirror 3 due to tolerances of the electric actuator.

Taking into account these approximations, the beam, reflected by the various primary mirrors 3, can be offset with respect to its optimum position, namely the centre of the absorbing tube 7. Moreover, the beam reflected by every single mirror has a greater surface than the tube one, so that part of the reflected sun energy would be lost. In order to allow the whole reflected sun radiation to cover the absorbing tube 7, a "non-imaging" type of secondary mirror 5' has been built, made of extruded aluminium, as shown in FIG. 17.

The secondary mirror 5' is composed of two extruded profiles 5a' put together and joined through bolts. Its particular shape is designed starting from the acceptance semi-angle θ and the receiver (absorption element) radius r. The secondary mirror 5 ¹ of the concentrator is made of an involute section and a macrofocal parabola section, as shown in FIG. 18.

The radiation that falls within the acceptance angle 2Θ is captured and concentrated on the receiver having radius r through a Compound Macrofocal Parabola Concentrator (CMPC) .

Regarding the computation of the above concentrator, firstly the involute included in VP is computed and then point P is used to compute the macrofocal parabola PQ.

The involute VP has the following equation: r(cαs(φ + α _j ) , sin(φ — O ₁ )) +rφ(cos(φ — π/2 + O ₁ ) ,sin(φ - π/2 +■ ctj))

With αj = -π/2 since the involute touches the circumference in point V that has an angle -π/2 with the horizontal axis xl, it becomes r(— φcosφ + srnφ, — cosφ — sinφ)

with -(π/2 + θ) ≤φ < 0 .

The section PQ is a macrofocal parabola rotated by an angle α= π/2-θ with respect to the horizontal axis. From point P the following values are obtained:

φp=π

From these previous values , constant K is computed through the following relationship : K = t _p — t _p cosφp + r +rφp — rπ/2 — rsinφp

or

Then the macrofocal parabola can be computed as follows:

K _ _r fφ _ _π /2) - r( 1 - sinΦI r(sm(φ + α),-cos(φ + α)) + 1 - cos _Φ ^(∞s(Φ +α),sm(φ + α))

then

— ^ — -(cosθ - cos(φ - θ) + (2π -φ + 2θ)sin(φ - θ),(-2π +φ - 2θ)cαs(φ - θ) - sm(φ - θ) - sinθ) cosφ — 1

with 2θ < φ <π. The previous process has therefore brought about the construction of the shape to be assigned to the secondary mirror 5' in order to obtain that sunrays that are incident and reflected by the primary mirror 3 are completely concentrated on the surface of the absorption element 7.

As regards the operating methodology of the secondary mirror 5' , on the lower part of the extruded profile 5a' a sheet with a solar grade is mechanically or chemically fastened, that increases the amount of energy reflected on the thermal absorption element 7. Such sheet is preferably made of an aluminium alloy or a technopolymer, namely a thermoplastic resin polymer.

The secondary mirror 5' will be worked as included below, so that thermo-insulating material (cupels) can be inserted into the junctions between a thermal absorption element 7 and a following thermal absorption element 7, and moreover at the ends of the sun receiver.

Figure 19 shows some of the technical solutions devised for connecting the two extruded profiles 5a ¹ made of aluminium, that together form the CMPC. Workings provide for through holes 28 used for joining the two extruded profiles 5a' with the help of a screw and nut, so that such elements can be inserted in the central parts of the extruded profile 5a ¹ ; a further working has been devised through orthogonal millings, that allow the operator to join the two extruded profiles 5a ¹ . The above hole 28 can have any shape, not only the circular shape shown.

Figure 20 shows the workings performed in order to be able to insert the insulating cupels in the junction between a secondary mirror 5 ¹ and the following one; the same workings are useful at the ends of the secondary mirror 5', where the absorption tube 7 that contains the thermo-vector element will be connected to the remaining part of the circuit.

Another feature of the thermal sun concentrator of the invention is that is has been designed and made in such a way as to offer a high modularity: this implies that it is possible to build systems of sun concentrators calibrated on the actual user needs. The sun concentrator of the invention is composed of elements that can be mutually connected in series and/or in parallel, and is therefore devised as kit that allows extending the system to its desired dimensions.