WO2010039035A2

WO2010039035A2 - Solar collector

Info

Publication number: WO2010039035A2
Application number: PCT/NL2009/050591
Authority: WO
Inventors: Robert Edwin De Jong
Original assignee: SOLDEQ SOLAR SYSTEMS BV
Current assignee: SOLDEQ SOLAR SYSTEMS BV
Priority date: 2008-10-01
Filing date: 2009-09-30
Publication date: 2010-04-08
Anticipated expiration: 2011-04-01
Also published as: NL2002960C2; WO2010039035A3; NL2002960A

Abstract

A device for converting solar radiation to heat comprises : - a mirror which in cross-section has the form of a parabola; and - a radiation absorption tube through which heat transport medium can flow and which extends in the longitudinal direction of the mirror through the focal point of the parabolic cross-section. The mirror comprises two mirror parts which are each mounted on either side for pivoting around an axis extending in longitudinal direction on a mirror carrier. The mirror parts are pivotable about a pivot axis between an operative position and a rest position in which the mirror parts are pivoted toward each other with their reflective surfaces such that the mirror is folded down.

Description

SOIAR COLLECTOR

The invention relates to a device for converting solar radiation to heat, comprising a mirror with a prismatic form which in cross-section has substantially the form of a parabola, a radiation absorption tube through which heat transport medium can flow and which extends in the longitudinal direction of the mirror through the focal point of the parabolic cross-section, and support means for supporting the mirror and the radiation absorption tube. Such a device is generally known, particularly in the form of large-scale, fixedly disposed solar collectors.

Increasing energy costs and increasing energy awareness have created a need to apply such solar collectors in more situations, including more small-scale and mobile situations. The size of prior art devices is however considerable. The invention has for its object to provide such a device which, at least during transport, has a smaller size.

The invention provides for this purpose a solar collector of the above stated type, wherein the mirror comprises at least two mirror parts which are each mounted on either side for pivoting around an axis extending in longitudinal direction on a mirror carrier situated on either side of the mirror parts and forming part of the support means, and wherein the mirror parts are pivotable about a pivot axis between an operative position, in which the mirror parts together form substantially a parabola in cross-section, and a rest position in which the mirror parts are pivoted toward each other with their reflective surfaces such that the mirror is folded down. Using these measures the size of the device can be considerably reduced when it is not in use, so that it can be displaced more easily.

The invention likewise relates to a method for converting solar radiation to heat, comprising of allowing sunlight to impinge upon a mirror with a prismatic form having in cross-section substantially the form of a parabola, and allowing a heat transport medium to flow through a radiation absorption tube extending in the longitudinal direction of the mirror through the focal point of the parabolic cross-section, wherein the mirror is initially carried from its rest position to its operative position by causing at least two mirror parts to pivot about a pivot axis.

In order to enable adjustment of the azimuth of the mirror to the changing position of the sun, it is attractive for the support means to comprise a substantially U-shaped frame and for the mirror carriers each to be connected to a ϋ-shaped frame forming part of the support means for rotation about the same azimuth axis differing from the pivot axes. The mirror can hereby be placed such that the axis of the parabolic cross-section of the mirror extends as parallel as possible to the direction of the solar radiation.

It is noted here that this axis preferably extends through or in the vicinity of the centre of gravity of the mirror.

The device preferably comprises azimuth drive means for causing rotation of the mirror about the azimuth axis, and azimuth control means for controlling the azimuth drive means so that in the operative position the axis of the parabolic cross-section of the mirror always extends as parallel as possible to the direction of the solar radiation. Adjustment of the azimuth of the mirror can hereby take place automatically. It is otherwise pointed out that the output of the solar collector greatly depends on the azimuth; only small variations result in a greatly reduced output.

The azimuth control means preferably make use of a direction-sensitive light sensor, whereby the control can be greatly simplified compared to for instance a time and position-dependent control.

According to a structurally simple embodiment, the light sensor comprises a plate to be directed with its main surface toward the sun and two luminous flux meters disposed on either side thereof, and the azimuth control means are also adapted to adjust the azimuth of the mirror when the luminous flux of the luminous flux meters differs.

This same embodiment provides a method wherein the mirror is rotated about an azimuth axis for the purpose of making as small as possible the shade on a surface from a body to be directed toward the sun. The output of a solar collector depends not only on the azimuth of the solar collector but also, albeit to a lesser extent, on the elevation. In order to also enable adjustment of the elevation, a further embodiment provides the measure that the U-shaped frame is mounted on a carriage forming part of the support means for rotation about an elevation axis extending transversely of the longitudinal direction of the U-shaped frame, and that the device comprises elevation drive means for causing rotation of the frame about the elevation axis.

For automatic adjustment of the elevation it is attractive that the device comprises an elevation control member for controlling the elevation drive means and that, depending on the geographical position and the date, the longitudinal direction of the mirror extends in the operative position as perpendicularly as possible to the incident solar radiation.

In order to improve the transportability of the device and to enable rotation of the longitudinal direction of the mirror as parallel as possible to the projection of the solar radiation on the earth's surface, it is recommended that the carriage is placed on a ground surface for rotation about a vertical axis. A separate frame can be present here on which the carriage can rotate, although it is likewise possible for the carriage to rest on the ground surface, for instance by means of swivel wheels.

Possible variations in the elevation of the mirror can be compensated with simple means when the absorption tube protrudes at both end zones beyond the edges of the mirror. The beams of light reflected due to the variation in the elevation of the mirror then nevertheless impinge upon the absorption tube.

Although the major part of the sunlight impinges upon the mirror in parallel beams and, due to the parabolic form of the mirror, is reflected to the absorption tube placed in the focal point, it is not always possible to avoid a part of the sunlight impinging upon the mirror from a different direction, for instance due to diffusion by a layer of cloud, and so not being reflected to the absorption tube. In order to nevertheless enable useful application of at least a part of this light, which is otherwise lost for the operation of the device, a preferred embodiment provides a secondary mirror which is adapted to direct onto the absorption tube solar radiation reflected by said first mirror but not directed toward the absorption tube. In order to make possible the diverse movements of the absorption tube relative to the fixed surface on which the device is placed, and where the connections for the efficient use of the heat transport medium are situated, it is recommended that use is made of a secondary mirror adapted to direct onto the absorption tube solar radiation reflected by said first mirror but not directed toward the absorption tube.

The absorption tube preferably has a cross-sectional form such that its effective absorbing outer surface is large compared to the inner cross-sectional throughflow surface in relation to the flow rate and the specific heat of the applied heat transport medium. The heat-receiving surface of the absorption tube is on the one hand hereby increased, but the thus captured heat is on the other hand concentrated in a smaller quantity of heat transport medium, so that it is heated to a higher temperature, this resulting in thermodynamic advantages .

Mirrors must be form-retaining in order to ensure reflection in the correct manner. The construction of the device according to the invention is simplified when each mirror part comprises a reflective layer on a preformed body of stable form.

Prior art mirrors therefore usually comprise rigid and heavy carriers on which the reflective layer is arranged. In order to obtain a construction which is easy to handle and light in accordance with the initially stated object of the present invention, it is recommended that the body of stable form consists substantially of a foam, for instance expanded polystyrene. The form-retention of the construction is further improved when the foam is received in a casing of stable form, for instance of a hard polyurethane such as Polymed®.

The applying of reflective layers in the prior art usually takes place on flat carriers. Particularly in the case of curved carriers according to the invention it is attractive for the reflective layer to be applied by vapour-deposition, sputtering or other suitable process.

Prior to applying of the reflective layer the carrier must be brought into the desired form. This can take place in attractive manner when the surface to which the reflective layer is applied is modelled by cutting with a tensioned wire, followed by an ageing, for instance for about six weeks, for the purpose of obtaining the desired stability of form. It is attractive for thermodynamic reasons that the radiation absorption tube is formed by a heat pipe. The heat obtained from the sun is hereby used not only to heat the heat transport medium, but also to cause a phase change, in particular evaporation, so that a larger amount of thermal energy can be absorbed.

The invention will be elucidated with reference to the accompanying drawings, in which: figure 1 is a perspective view of a solar collector according to the invention in a first operative position; figure 2 is a perspective view of the solar collector shown in figure 1 in a second operative position; figure 3 is a perspective detail view of figure 2; figure 4 shows a view corresponding to figure 2 of a variant in which the elevation can be set manually in discrete positions; figure 5 shows a perspective detail view of figure 4; figure 6 is a perspective view of the solar collector shown in figures 1, 2 and 4 in the folded-down position; figure 7 shows a view corresponding to figure 2 of the solar collector, in which a cover is removed in order to show the azimuth adjusting device; figure 8 shows a cross-sectional view of a sensor of an azimuth control device; figure 9 is a perspective view of an alternative embodiment of the solar collector, which is provided with a secondary mirror; figure 10 shows a cross-sectional view of a secondary mirror with an insulating layer and a round absorption tube; figure 11 shows a cross-sectional view of an alternative embodiment of an absorption tube formed by a profile which is to some extent triangular; and figure 12 shows a cross-sectional view of an alternative embodiment of a mirror .

Figures 1 and 2 show a first embodiment of a device according to the invention. The device, designated in its entirety with ^Λl', comprises a carriage 2 which is manufactured for instance from metal profiles, extends substantially horizontally and rests on a ground surface 4 by means of swivel wheels 3. The carriage can be fixed on the ground surface by means of jacks 5. A ϋ-shaped frame 10 comprising a base 11 and two legs 12, 13 is mounted for rotation with its base 11 on a shaft

14 extending horizontally through the carriage. It is hereby possible to adjust the angle of elevation of U-shaped frame 10. For the purpose of determining and fixing the angle of elevation an end of a sub-frame 15 is connected rotatably to base 11 of ϋ-shaped frame 10. The other end of sub-frame

15 is connected slidably to a transverse beam of carriage 2. Sliding the end of the sub-frame relative to the transverse beam changes the angle of elevation of U-shaped frame 10.

A respective yoke 16, 17 is connected to the distal end of legs 12, 13 of the ϋ-shaped frame for rotation about an axis extending parallel to the base. Both yokes 16, 17 are thus rotatable about this axis. A drive mechanism, which will be elucidated below, is placed in both legs 12, 13 of the U-shaped frame. Both drive mechanisms are adapted to move yokes 16, 17 simultaneously. The device also comprises two mirror parts 20, 21, both mounted rotatably on both yokes

16, 17. Both mirror parts 20, 21 are thus supported on each side by a yoke 16, 17, wherein mirror parts 20, 21 are rotatable about the same axis relative to the two yokes 16,

17. Both mirror parts 20, 21 have in cross-section the form of a parabola. An absorption tube 22 moreover extends between the two yokes 16, 17. The form of mirror parts 20, 21 is such that in the position shown in figures 1 and 2 they together have the cross-sectional form of a parabola, wherein absorption tube 22 is located in the focal point.

As shown in the drawings, the mirror parts have a great thickness compared to prior art mirrors. This is the result of the construction of the mirror parts, which are each formed by a basis of a light material, such as expanded polyethylene, which is preferably enclosed by a layer of a rigid material such as polyurethane. A reflective layer, for instance of aluminium, is vapour-deposited on the reflective side. For the purpose of driving the mirror in a direction for changing the elevation use is made of an electric motor 30 as shown in figure 3. Using two ears 31 this electric motor 30 is mounted rotatably on the transverse beam of carriage 2. The output shaft of electric motor 30 forms a screw spindle of an adjusting device provided with a screw spindle and a nut, wherein the nut is connected to sub-frame 15. By switching on electric motor 30 the distance between the transverse beam of carriage 2 and the shaft with which sub-frame 15 is connected to base 11 of U-shaped frame 10 is thus changed, whereby the elevation of U-shaped frame 10, and thereby of mirror parts 20, 21, is changed.

Figure 4 shows an alternative embodiment wherein sub-frame 15 is provided with pins 18 which can be inserted into holes 19 arranged in longitudinal beams of the carriage. This is an operation to be performed by hand, so cannot be automated. The elevation is also limited to a discrete number of positions. This construction shown in more detail in figure 5.

As already elucidated with reference to figure 1, mirror parts 20, 21 can each be tilted on a shaft relative to yokes

16, 17. This provides the option of allowing mirror parts 20, 21 to tilt toward each other to the position shown in figure 6. Despite their curved form, mirror parts 20, 21 extend here as parallel as possible to each other so that they take up as little space as possible.

Yokes 16, 17 are also rotated through an angle of about 180° so that the mirror parts extend downward from yokes 16,

17. The height of the whole installation is hereby reduced and the centre of gravity is displaced downward so that the device is more stable and can be more easily displaced and stowed away.

Figure 7 corresponds to figure 2, except that a cover forming part of leg 12 of U-shaped frame 10 is removed, and the interior of this leg is thus visible. Shown here is that yoke 16 is connected to a belt pulley 35 by means of a shaft extending through a part of leg 12. This pulley is connected by means of a preferably toothed belt 36 to a second pulley 37 placed on an output shaft of an electric motor (not shown} accommodated in base 11 of the U-shaped frame. The position of yoke 16 relative to leg 12 can be changed by rotation of the electric motor. It will be apparent that a corresponding drive device is placed in the other leg 13 of the U-shaped frame and that both drive devices are used simultaneously to cause simultaneous rotation of both yokes 16, 17. Instead of two electric motors each driving a single yoke, a single electric motor can be used which, for instance via a long shaft, can drive both yokes simultaneously.

A drive can also be provided to cause rotation of mirror parts 21, 22 relative to yokes 16, 17, although it is likewise possible for the change in the position of the mirror parts to be carried out by the user. The mass of the mirror parts is after all relatively small, as will be apparent from the following, and movement need only take place between the folded-down position, which can be easily defined by stops, and the outward folded position, which can also be defined by stops or which is defined by cables 38 as shown in figure 7. Other possibilities for defining the extreme positions are however by no means precluded. Nor is the presence precluded of a separate drive device for driving the mirror elements between their extreme positions. For the purpose of controlling the rotation of mirror parts 20, 21 together and yokes 16, 17 together about the axis extending through the distal ends of legs 12, 13, in the present case for controlling the electric motor, use is made of a sensor designated in its entirety with 40 as shown in figure 8. This sensor 40 is placed on one of the yokes 16, 17. The sensor comprises two light-sensitive elements 41 and a plate extending perpendicularly of and between these elements. The sensor also comprises a comparing circuit (not shown) . The sensor is placed here such that the plate 42 extends parallel to the axis of symmetry of mirror parts 20, 21. When the mirror parts have taken up their optimal azimuth position, i.e. with their axis of symmetry parallel to the direction of the solar radiation, both light-sensitive elements 41 will then receive the same amount of light. This is because they are also placed symmetrically. As the position of the sun changes through time, the direction of the solar radiation will change, thereby creating an asymmetry, and one of the two light-sensitive elements will receive less light. In response the comparing circuit will activate the electric motor, whereby the position of the mirror halves and that of the sensor will change, this to the position at which symmetry is restored. An optimum position is hereby maintained at all times. The sensor has the advantage here over the collimators that are applied because the present sensor not only indicates asymmetry but also to which side the mirror must be moved in order to restore symmetry.

Figure 9 shows the device according to the invention in a rather extreme position of the mirrors for the purpose of obtaining an optimum position of the mirror parts at dusk, i.e. in a low position of the sun.

Figure 10 shows a cross-section of an absorption tube 22. This comprises the actual absorption tube 45 of substantially quadrilateral cross-section and a mirror part 46 placed above absorption tube 45 and adapted to reflect light which is reflected by the mirrors and which would otherwise have missed absorption tube 45. Supports 47 are also arranged for supporting mirror 46.

Figure 11 shows an embodiment of absorption tube 48, the internal section of which differs from a circle. The surface area with which heat transfer to the heat-absorbing liquid takes place is hereby enlarged. The same applies for the external section.

Finally, figure 12 shows a cross-section of mirror parts 20, 21. This shows that the mirror parts have a great thickness, this being the result of the fact that they comprise a core 50 of a light material such as expanded polystyrene which, in order to increase the form-retention, is provided with a casing 51 of a form-retaining material such as polyurethane. In order to obtain the mirror effect a reflective layer 52 is applied to the relevant surface of the casing.

It will be apparent that diverse variations can be made to the shown embodiments. It is also possible to apply diverse measures without other measures described in the foregoing.

Claims

CIAIMS

1. Device for converting solar radiation to heat, comprising:

- a mirror with a prismatic form which in cross-section has substantially the form of a parabola;

- a radiation absorption tube through which heat transport medium can flow and which extends in the longitudinal direction of the mirror through the focal point of the parabolic cross-section; and

- support means for supporting the mirror and the radiation absorption tube, characterized in that the mirror comprises at least two mirror parts which are each mounted on either side for pivoting around an axis extending in longitudinal direction on a mirror carrier situated on either side of the mirror parts and forming part of the support means, and that the mirror parts are pivotable about a pivot axis between an operative position, in which the mirror parts together form substantially a parabola in cross-section, and a rest position in which the mirror parts are pivoted toward each other with their reflective surfaces such that the mirror is folded down.

2. Device as claimed in claim 1, characterized in that the support means comprise a substantially U-shaped frame and that the mirror carriers are each connected to a U-shaped frame forming part of the support means for rotation about the same azimuth axis differing from the pivot axes.

3. Device as claimed in claim 1 or 2, characterized by azimuth drive means for causing rotation of the mirror about the azimuth axis, and azimuth control means for controlling the azimuth drive means so that in the operative position the axis of the parabolic cross-section of the mirror always extends as parallel as possible to the direction of the solar radiation.

4. Device as claimed in claim 3, characterized in that the azimuth control means comprise a direction-sensitive light sensor.

5. Device as claimed in claim 4, characterized in that the light sensor comprises a plate to be directed with its main surface toward the sun and two luminous flux meters disposed on either side thereof, and that the azimuth control means are also adapted to adjust the azimuth of the mirror when the luminous flux of the luminous flux meters differs.

6. Device as claimed in any of the claims 2-5, characterized in that the U-shaped frame is mounted on a carriage forming part of the support means for rotation about an elevation axis extending transversely of the longitudinal direction of the U-shaped frame, and that the device comprises elevation drive means for causing rotation of the frame about the elevation axis.

7. Device as claimed in claim 6, characterized in that the device comprises an elevation control member for controlling the elevation drive means and that, depending on the geographical position and the date, the longitudinal direction of the mirror extends in the operative position as perpendicularly as possible to the incident solar radiation.

8. Device as claimed in claim 6 or 7, characterized in that the carriage is placed on a ground surface for rotation about a vertical axis.

9. Device as claimed in any of the foregoing claims, characterized in that the absorption tube protrudes at both end zones beyond the edges of the mirror.

10. Device as claimed in any of the foregoing claims, characterized by a secondary mirror which is adapted to direct onto the absorption tube solar radiation reflected by said first mirror but not directed toward the absorption tube.

11. Device as claimed in any of the foregoing claims, characterized in that the support means are provided with two high-torsion compliance tubes or hoses which connect the ends of the absorption tube to conduits which are fixedly disposed, which conduits are adapted to respectively supply and discharge the heat transport medium to be guided through the absorption tube.

12. Device as claimed in any of the foregoing claims, characterized in that the absorption tube has a cross-sectional form such that its effective absorbing outer surface is large compared to the inner cross-sectional throughflow surface in relation to the flow rate and the specific heat of the applied heat transport medium.

13. Device as claimed in any of the foregoing claims, characterized in that each mirror part comprises a reflective layer on a preformed body of stable form.

14. Device as claimed in claim 13, characterized in that the body of stable form consists substantially of a foam, for instance expanded polystyrene.

15. Device as claimed in claim 14, characterized in that the foam is received in a casing of stable form, for instance of a hard polyurethane such as Polymed®.

16. Device as claimed in claim 13, 14 or 15, characterized in that the reflective layer is applied by vapour-deposition, sputtering or other suitable process.

17. Device as claimed in any of the claims 13-16, characterized in that the surface to which the reflective layer is applied is modelled by cutting with a tensioned wire, followed by an ageing, for instance for about six weeks, for the purpose of obtaining the desired stability of form.

18. Device as claimed in any of the foregoing claims, characterized in that the radiation absorption tube is formed by a heat pipe.

19. Method for converting solar radiation to heat, comprising of:

- allowing sunlight to impinge upon a mirror with a prismatic form having in cross-section substantially the form of a parabola; and - allowing a heat transport medium to flow through a radiation absorption tube extending in the longitudinal direction of the mirror through the focal point of the parabolic cross-section, characterized in that the mirror is initially carried from its rest position to its operative position by causing at least two mirror parts to pivot about a pivot axis.

20. Method as claimed in claim 19, characterized in that the mirror is rotated about an azimuth axis on the basis of making as small as possible the shade on a surface from a body to be directed toward the sun.