Panel-like structure for collecting radiant energy

PANEL-LIKE STRUCTURE FOR COLLECTING RADIANT ENERGY

0002The present invention relates to structures for collecting radiant energy, more especially, (but not exclusively), solar energy and, in particular, to panel-like structures suitable for use in a wide variety of locations.

0003Solar energy is increasingly being used to supplement and, in some circumstances, replace other forms of energy in the production of electricity. However, although solar energy offers advantages from an environmental point of view it is still, for many applications, too expensive to justify consideration. For maximum efficiency, the collection of solar energy for conversion into electricity is carried out using an array of photovoltaic cells which tracks the sun to ensure that, throughout the year, it is always oriented to collect the greatest amount of energy. The installation and maintenance of such an array is, however, comparatively complex. Fixed arrays are easier to install and less costly but are less effective unless they are aimed in the direction of the average position of the sun (which is not always possible) or a very large area of photovoltaic cells is provided (in which case the cost of the photovoltaic cells can become prohibitively high).

0004It is recognized that one way of reducing the comparative cost of electricity produced from solar energy is to reduce the number of cells that is required to collect a given amount of solar energy. Various arrangements have been proposed with that in mind, and some examples are described in US-A-4 235 643; 4 313 023; 4 514 040; 5 419 782; and 5 466 301, and in SU-A-1 089 365.

0005In addition, a paper entitled "Ideal Prism Solar Concentrators" by D. R. Mills and J. E. Guitronich published in 1978 in "Solar Energy", Volume 21 at pages 423 to 430 describes the possibility of using simple prism units as solar concentrators, and of combining low concentration prism units into stationary, flat-top concentrators. A related disclosure appears in AU-A-37217/78.

0006The present invention is concerned with improving the efficiency and increasing the versatility of panel-like structures used for collecting radiant energy, more especially (but by no means exclusively) structures for use in buildings, road signs and hoardings.

0007The present invention provides a structure for collecting radiant energy, in accordance with claims 1 and 2 herein.

0008The present invention also provides a structure for collecting radiant energy in accordance with claim 9 herein.

0009The present invention further provides a structure for collecting radiant energy, in accordance with claims 16 and 17 herein.

0010By way of example only, embodiments of the invention will be described with reference to the accompanying drawings, in which:

0011Fig. 1 is a vertical cross-section through a panel-like structure that can be used for collecting radiant energy;

0012Fig. 2 is a diagram illustrating the location of various parts of the structure, as viewed in the direction of the arrow II in Fig. 1 ;

0013Fig. 3 is a diagrammatic vertical cross-section corresponding to Fig. 1, showing various ray paths through the structure; Fig. 4 is an enlarged version of part of Fig. 3, used for calculating the dimensions of parts of the structure;

0014Figs. 5 and 6 are views, similar to Fig. 4, illustrating structures in accordance with one aspect of the present invention;

0015Figs. 7(a) and (b) illustrate modified forms of the structure shown in Fig. 1 ; Figs. 8(a) and (b) are vertical cross-sections through a structure in accordance with another aspect of the invention, in combination, respectively with a window and a graphic panel;

0016Fig. 9 illustrates a further modification of the structure of Fig. 7(b); Figs. 10(a) and (b) and 11 illustrate alternative orientations of a structure for collecting radiant energy, in accordance with a further aspect of the invention; and

0017Figs. 12 to 14 are diagrammatic vertical cross-sections through other panel-like structures that can be used for collecting radiant energy.

0018Figs. 1 and 2 show a panel-like structure 1, suitable for use as part of a fixed construction, for collecting solar energy to be used for generating electricity. As explained in greater detail below, the structure 1 can be used in many different locations, including: on the inside surface of a window; on the outside of a building; and on road signs and hoardings.

0019The structure 1 comprises a vertical panel 2 of a light-transmitting material, having first and second major surfaces 3, 4. The first major surface 3 is planar while the second major surface 4 is profiled and comprises several elongated, generally prismatic structures 5 extending horizontally across the panel 2 one above the other. The prismatic structures 5 are identical, and run parallel to each other. Each prismatic structure 5 comprises mutually inclined faces 6, 7: the face 6 is a reflecting surface facing into the panel 2, and the face 7 carries a photovoltaic cell 8 which is in optical contact with the panel material to receive radiation transmitted through the latter. The photovoltaic cell 8 is in the form of a strip which covers the whole width of the face 7. Typical dimensions for the widths of the faces 6 and 7 are 10mm and 3mm respectively.

0020The panel 2 may be formed from a polymeric material, and may be a moulded structure. Suitable polymeric materials include polycarbonate, silicone, and acrylic materials, and fluoropolymers, for example Dyneon™ THV fluorothermoplastic material, available from Minnesota Mining and Manufacturing Company, St. Paul, Minnesota, USA and polytetrafluoroethylene (PTFE).

0021Fig. 3 illustrates the structure 1 positioned so that the vertical planar surface 3 faces the direction in which the sun attains its highest altitude (i.e. due south in the northern hemisphere). Electromagnetic radiation from the sun enters the panel 2 through the surface 3, as represented by the ray paths 10¹ to 10⁵, and some of that radiation (exemplified by the ray 10¹) will impinge directly on one of the photovoltaic cells 8 while the remainder will strike one of the reflecting faces 6 and be directed back into the panel. Radiation that is reflected from a face 6 close to the apex of the respective prismatic structure 5 will be directed to the adjacent photovoltaic cell 8, as exemplified by the ray IO² in Fig. 3, while the remainder of the reflected radiation will be directed back towards the planar surface 3, as exemplified by the rays 10³ to IO⁵. The angle at which the rays 10³ to IO⁵ strike the surface 3 within the panel depends, for a given angle of incidence from the sun, on the angle of inclination of the reflecting face 6 and the latter is selected to ensure that all of the reflected rays 10³ to 10⁵ will undergo total internal reflection (TIR) at the surface 3 and be returned yet again into the panel 2, as illustrated. This time, some of the TIR radiation will impinge on one of the photovoltaic cells 8, either directly as exemplified by the ray IO³ or after one more reflection at a face 6 as exemplified by the ray IO⁴. The remaining radiation, exemplified by the ray 10⁵, will be directed yet again to the surface 3 of the panel where it will undergo total internal reflection yet again and eventually reach a photovoltaic cell 8 by repetition(s) of the process just described (unless it encounters the edge of the panel 2 first).

0022Various configurations of the structure 1, in accordance with different aspects of the present invention, will be described below. In the description, reference will be made to the dimensions of various parts of the structure, those dimensions being as indicated in Fig. 4. To minimize the width S of the faces 7 and thus the total area of the solar cells 8 used in the structure 1 (i.e. a structure with planar faces 6, 7), the apex angle D of each prismatic structure 5 should be 90° and the angle of inclination T of each of the reflecting faces 6 to the general plane of the panel 2 (in this case, the vertical) should be as small as possible consistent with ensuring that all radiation which is returned to the surface 3 of the panel 2 undergoes total internal reflection as described above. The concentration ratio of the structure (defined as the ratio of the width W in the vertical direction of one of the prismatic structures 5 to the width S of a photovoltaic cell 8) will then be at a maximum. With the angle D set at 90°, it can be shown that the minimum value of the angle T is determined by the expression: sin 2T = sin C Λ/(1 - k²) - k cos C (i) where k = sin A/n

0023C is the critical angle for the material of the panel 2 n is the refractive index of the material of the panel 2, and A is the minimum sun altitude (in the vertical plane normal to the panel) at which energy is to be collected. Using expression (i) for a panel material having a refractive index of 1.5, and assuming that solar energy is to be collected at all times of the year, the minimum values shown in Table 1 below can be derived for the angle T over latitudes 40° to 60° (the values being shown with the accompanying concentration ratios of the structure 1). In deriving these values, the minimum sun altitude A is taken as the altitude of the sun at noon on 21^st December at each latitude.

TABLE 1

0025<img file="WO0077458A1_D0001.tif" /><img file="WO0077458A1_D0002.tif" />

0026A fixed panel-like structure of the type described above with reference to Figs. 1 and 2 can, through an appropriate selection of the angle T, collect substantially more (typically over three times as much) photovoltaic energy per unit area of photovoltaic cell than a plane panel of the same materials. Even if the angle T does not have the minimum possible value for the particular latitude at which the panel is used, an advantageous amount of solar energy can still be collected.

0027As an example of the above, if a structure as shown in Figs 1 and 2 is formed from a material having a refractive index n of 1.5 and with the angle T equal to 15.75° and is located (facing due south) at latitude 50° north then, assuming that the reflectivity of the faces 6 is 100% and that substantially all of the radiation directed to a face 7 actually enters the associated photovoltaic cell 8, it can be expected that the structure could show a net gain of about 3.6 (i.e. it could collect about 3.6 times as much solar energy per unit area as a plane photovoltaic cell potted in a material having a refractive index of 1.5).

0028Moreover, that increase in the amount of energy collected remains comparatively constant throughout the day and throughout the year.

0029The concentration ratio W/S of the structure shown in Fig. 1 varies with the index of refraction of the material of the panel 2 and, in particular, can be increased at any given latitude by employing a material having a greater index of refraction. That is illustrated by the following table, for latitude 50°.

TABLE 2

0031<img file="WO0077458A1_D0003.tif" /><img file="WO0077458A1_D0004.tif" />

0032It is also possible, in accordance with one aspect of the present invention, to achieve an increase in the concentration ratio W/S by modifying the shape of the prismatic structures 5, in particular the shape of the reflecting surfaces 6, as will now be described with reference to Figs. 5 and 6.

0033Fig. 5 is a view similar to Fig. 4, illustrating that the reflecting faces 6 of the prismatic structures 5 need not be completely planar. Each of the reflecting faces has an initial planar section 11 but comprises also a curved section 12 which commences at the point indicated by the reference P and joins the planar section to the apex of the prismatic structure 5 (and to the associated face 7). The point P is the location, in the face 6 of Fig. 4, at which a reflected ray from the sun at minimum altitude A grazes the edge 13 of the face 7. The position of the point P defines the width W, in the vertical direction of the planar section 11 of the reflecting face 6, which is given by the expression

0034W_j = W/ { l+(tan T) tan (B+2T) . The section 12 follows a parabolic curve (shown

0035continued in dotted lines beyond the apex of the prismatic structure) having the following characteristics: the focus of the parabola is located at the edge 13 of the face 7; the axis 14 of the parabola (also shown in dotted lines) is parallel to the initial path of a ray within the panel 2 when the sun is at the minimum altitude A (see above); and the origin-focus distance a of the parabola (i.e. the length of the axis 14 shown in Fig. 5) is given by the expression a = 0.5{M + V(M² + N²)} (ii) where M = L sin B

0036L - { (W, tan T)/tan β} - W + W, N = (W, tan T)/Sin B - L cos B

0037By shaping the reflecting face 6 as shown in Fig. 5, the widths of the photovoltaic cell 8 can be reduced (for a given width W of the prismatic structure 5) while still ensuring that all radiation incident on the reflecting face 6 below the point P (i.e. the parabolic section 12) will be directed onto the photovoltaic cell. Radiation that is incident on the reflecting face 6 above the point P (i.e. the planar section 11) will be directed back to the front face 3 of the structure 1 and will be totally internally reflected at that face as described above with reference to Fig. 3. The effect of reducing the width S of the photovoltaic cell is to increase the concentration ratio of the structure, and the greatest effect is achieved by arranging the face 7 to lie along the axis 14 of the parabolic curve 12. In that case, the width of the photovoltaic cell 8 will be as small as possible but, nevertheless, will receive all radiation incident on the parabolic section 12 of the reflecting surface. However, it may be more practical (from a manufacturing viewpoint) to arrange the face 7 at 90° to the front face 3 of the panel 2 as shown in Fig. 5 although the concentration ratio of the structure will be slightly lower.

0038Fig. 6 illustrates an alternative prismatic structure 5, in which the parabolic section 12 of Fig. 5 is replaced by two mutually-inclined planar sections 15¹, 15² one of which (15¹) is a continuation of the planar section 11 of the reflecting face 6. The planar sections 15¹, 15² together provide a shape that corresponds substantially to that of the parabolic section 12 and necessitate some adjustment in the width and orientation of the face 7 carrying the photovoltaic cell 8 but the planar section 11 of the reflecting face 6 (above the point P) remains unchanged. The configuration illustrated in Fig. 6 offers a slightly lower concentration ratio for the panel 2 than that of Fig. 5 but, unlike the Fig. 5 configuration, does not result in all of the minimum angle radiation (i.e. radiation incident on the front face 3 at the angle A) reflected from the lower part of the face 6 being brought to a focus on the edge 13 of the photovoltaic cell 8 when the sun is at the minimum altitude A: instead, some of that radiation will now be directed to other areas of the photovoltaic cell. If desired, the parabolic section 12 of Fig. 5 could be replaced by more than two mutually- inclined planar sections (one being a continuation of the section 11, in the manner of section 15¹ in Fig. 6), likewise providing a shape that corresponds substantially to that of the parabolic section.

0039In each of the configurations described above, the photovoltaic cells 8 may be any suitable type in the form of a strip, and are located on the faces 7 so that they are in optical contact with the material of the panel 2. In the case in which the panel 2 is formed by moulding, the required optical contact could, for example, be achieved by making the photovoltaic cells an integral part of the moulded product.

0040The reflecting surface on the faces 6 of the panels 2 can be formed by depositing a reflective material on those sections or by attaching a pre-formed reflective material to those sections. Advantageously, the reflecting surface has a reflectivity of at least 90% and an example of a suitable pre-formed material is a silver reflective film available, under the trade designation "Silverlux", from Minnesota Mining and Manufacturing Company of St. Paul, Minnesota, USA. That material may be laminated to the panel 2 after the latter has been formed or, in the case in which the panel is formed by moulding, may be an integral part of the moulded product. Examples of other pre-formed reflective materials suitable for use on the faces 6 of a panel are described in US-A-5 882 774 and WO

004197/01774. Further examples are reflective films available, under the trade designations "ECP 305 A" and "SA-85P" from Minnesota Mining and Manufacturing Company; a silvered film on aluminium available, under the trade designation "EVERBRITE 95", from Alcoa of Sidney, Ohio, USA; a silver film available, under the trade designation "SPECULAR PLUS", from MSC Laminates &amp; Composites Inc., of Elk Grove Village, Illinois, USA; and aluminium reflective sheeting available, under the trade designation "MIRO 4" from Alanod Aluminium- Veredlung GmbH &amp; Co, of Ennepetal, Germany. An alternative form of the panel of Fig. 1, in which the reflective material 16 is spaced apart from the faces 6, is illustrated in Fig. 7(a). The reflective material 16 is shown positioned parallel to the respective face 6 but space apart from it by an air gap 17 which is closed at both ends by the adjacent photovoltaic cell 8. In this case, the presence of the air gap 17 will cause some of the radiation incident on the face 6 (represented by the ray 18) to be totally internally reflected and thus directed back into the panel without any absorption losses. The remaining radiation incident on the face 6 (represented by the ray 18¹) will be directed back into the panel either by partial reflection at this face ( as indicated by the ray 18²) or by reflection, with absorption losses, at the material 16 (as indicated by the ray 18³).

0042In Fig. 7(a), the air gap 17 could be replaced by a solid layer of a material having a low index of refraction in comparison to the material of the panel 2. In that case, the solid layer will provide support for the reflective material 16. Although the arrangement shown in Fig. 7(a) does not offer an appreciable advantage over the arrangements of Figs. 1 to 6 if the material 16 has a very high reflectivity, it will give an improved performance if the reflectivity of the material 16 is less good.

0043In some circumstances, it is possible to omit completely the reflective material associated with the faces 6 of the panel of Fig. 1, as illustrated in Fig. 7(b). In that case, only radiation (exemplified by the ray 19) that undergoes total internal reflection at the faces 6 will be redirected into the panel 2 and may eventually impact one of the photovoltaic cells 8: the remainder of the radiation (other than that which falls directly on the photovoltaic cells) will simply pass out of the panel 2 through the faces 6 and be lost, as indicated by the ray 19¹. Through a suitable choice of the angle T (see Fig. 3), in accordance with: n sin (C-T) = sin A (iii)

0044(where n, A and C are as defined above) it can, if desired, be ensured that all solar radiation falling on the faces 6 undergoes total internal reflection, although the concentration ratio W/S of the structure 1 will be lower. On the other hand, because of the simplicity of the structure when the reflective material is omitted, it may be acceptable in certain circumstances to use a panel having a lower concentration ratio despite the losses due to radiation passing out of the panel through the faces 6. By way of example, reference can be made again to the specific structure (mentioned above) of the type shown in Figs. 1 and 2 (i.e. a structure formed from a material having a refractive index of 1.5 and with the angle T equal to 15.754°, and located facing due south at latitude 50°N). As stated above, if the reflectivity of the faces 6 is 100%, it can be expected that such a structure would show a constant net gain of about 3.6. If, for comparison, the reflective material is then removed from the faces 6 so that the structure becomes of the type shown in Fig. 7(b), the net gain will remain substantially the same in mid-summer but will be lower at other times of the year (varying from a comparatively constant value of about 0.54 throughout the day in mid- winter to an average daily value of about 2.85 (with wide variations) around the vernal and autumnal equinoxes).

0045The alternative panel 2 of Fig. 7(b) can be used to advantage on the inside of window glass 20 as illustrated in Fig. 8(a). In that case, in addition to collecting solar energy which passes through the glass, the panel 2 will eliminate direct glare from the sun (due to the presence of the photovoltaic cells 8) while still allowing an observer 21 inside the building to see through the window (via the surfaces 6) to the ground outside.

0046Fig. 8(b) illustrates that the panel 2 of Fig. 7(b) can also be used to advantage in front of a graphic panel 30 which is intended to be viewed from below (i.e. by an observer 31 positioned to look up towards the graphic panel 30). The graphic panel 30 will be visible to the observer through the planar faces 6 of the panel 2 but the latter will, nevertheless, also function to collect solar energy which may, in turn, be used to generate electricity to store in a battery for illuminating the graphic panel (which may, for example, be a back-lit sign). The panel 30 may be located on the outside of a building, or it may be any other form of vertical panel such as an advertising hoarding or a road sign. Since the front face of the graphic panel 30 will be protected by the panel 2, it is possible to construct the graphic panel from materials which, otherwise, would not be able to withstand the effects of the weather.

0047Fig. 9 illustrates a modification to the panel 2 of Fig. 7(b), for use in eliminating distortion of an image that is viewed through the panel. The modification comprises the provision, adjacent the profiled face 4 of the panel 2, of a second panel 33 formed from the same material as the first panel to compensate for the refraction of light rays passing through the latter. The face 35 of the second panel 33 adjacent the panel 2 has a profile which is complementary to that of the face 4, and there is a small air gap 37 between the two faces to ensure that the amount of radiation directed to the photovoltaic cells 8 remains unchanged. To an observer 38 looking through the panels, however, the assembly 2, 33 functions as if it were a parallel plate and any distortion of the view that might have been apparent in the absence of the second panel 33 is eliminated.

0048In the above description, it is assumed that the panel 2 is positioned so that the vertical front face 3 faces in the geographical direction in which the sun attains its highest altitude (i.e. due south in the northern hemisphere). The panel 2 can also be used to advantage (i.e. without a substantial reduction in the amount of solar radiation that it can collect) when rotated up to about 45° to the east or west provided that the structure is rotated in its own plane at the same time and the angle T is increased slightly to ensure the collection of solar energy when the sun is at minimum altitude. In other words, the front face 3 remains vertical but the prismatic structures 5 no longer run horizontally. This is illustrated in Figs. 10(a) and (b) which are diagrammatic views from the front and from above, respectively, of a south-facing panel 22, a south-east-facing panel 23, and a southwest-facing panel 24. The panels 23 and 24 have been rotated clockwise and anticlockwise, respectively, relative to the panel 21 as seen in Fig. 9(a). In one practical example, a panel of the type shown in Figs. 1 and 2, formed from a material having a refractive index of 1.5, is used in a vertical orientation facing 30 east of south at a latitude of 50°N. The panel is rotated clockwise through an angle R of 24.94° (as viewed in the direction of the arrow II in Fig. 1) and the angle of inclination T of the reflecting facets is set at 17.7° (giving a concentration ratio W/S of 3.29). The same panel will, of course, function satisfactorily facing 30° west of south at the same latitude, but should be rotated through 24.94° in an anti-clockwise, rather than a clockwise, direction. Other practical examples are summarized in Table 3 below. In each case, the panel (which is of the type shown in Figs. 1 and 2) is formed from a material having a refractive index of 1.5 and is used in a vertical orientation at a latitude of 50°N.

0049<img file="WO0077458A1_D0005.tif" />

0050Any of the other panels described above (i.e. with reference to Figs. 5 to 9) can also be used with advantage in the manner illustrated in Fig. 10. In each case, the panel is rotated in its own plane so that the prismatic structures 5 are inclined to the horizontal, and the angle T is increased to ensure collection of solar energy when the sun is at minimum altitude.

0051It has also been assumed in the above description that the panel 2 is vertical. The same structure will, however, also function in a non- vertical orientation, for example on the inclined (pitched) roof of a house or as part of a collector array on the ground, and may still offer advantages in comparison with a conventional plane, non-tracking, solar panel oriented in the optimum direction. The optimum direction is the direction of the average position of the sun (taken to be the position of the sun at mid-day on the equinox), and a panel 25 that is oriented in that way is shown in Fig. 11 with the average position of the sun being indicated at 26. At latitude 50°, the optimum aim altitude for the panel 25 would be 40°. A structure of the type shown in Fig. 1, used at this latitude and oriented in this way can, provided it is adapted to receive minimum angle radiation from the sun (i.e. radiation from the December sun), collect twice as much solar energy over the course of a year as a plane panel for the same area of photovoltaic cells. The net gain is, moreover, comparatively constant throughout the year. At lower aim altitudes (i.e. less than 40°) the amount of solar energy collected by the Fig. 1 type of structure over the course of a year can be increased even further but exhibits wider variations throughout the year, with the highest gain relative to the conventional panel 25 of Fig. 11 being achieved in the winter months. Any of the other panels described above, with reference to Figs. 5 to 9, can also be used in a non-vertical orientation.

0052Figs. 12 and 13 illustrate further modifications of the structure shown in Fig. 1. The panel-like structure 27 shown in Fig. 12 comprises, effectively, a single one only of the prismatic structures of Fig. 1. In this case, the geometry of the structure is selected to ensure that at least part of the radiation that is reflected from the face 6 will reach the photovoltaic cell 8 directly while another part will reach the solar cell after total internal reflection at the front face 3. The structure illustrated in Fig. 12 comprises a series of single prismatic structures 27, each of the type shown in Fig. 11, assembled one adjacent another to form a larger panel. It will be appreciated that any of the other prismatic structures described above with reference to Figs. 5 to 8 could also be used in the manner illustrated in Figs. 12 and 13.

0053Any of the panel structures described above could be laminated to a transparent panel in the manner illustrated in Fig. 8(a). The transparent panel need not be a window in a building but could, for example, serve the purpose of protecting the panel from the environment or against physical damage when it is used in an exposed situation. In the above description with reference to the drawings, it has been assumed that the radiant energy collectors are in the form of photovoltaic cells. Other forms of energy collector could be used, however, including heat collectors. Fig. 14, for example, shows a panel structure of the type shown in Fig. 1, in which the photovoltaic cells 8 are replaced by heat-collecting tubes 28. If the panel is a moulded structure, the tubes 28 may be incorporated during the moulding operation. The tubes 28 may contain water or another suitable fluid to transport heat, or they may be heat pipes (i.e. pipes that transfer heat through repeated vaporization and condensation of a fluid).

0054In the above description of panels that comprise a plurality of prismatic structures, it has been assumed that the prismatic structures are identical. That is not essential, however, provided that the various mechanisms by which radiation is directed to the collectors 8, 28 in the panel are still present. It is also not essential for the front face 3 of the panel to be exactly planar: in some circumstances, it may be desirable to provide the panel with a front face which is structured (although on a significantly smaller scale than the rear face 4), for example to reduce radiation loses through partial reflection at the front face. It may also, in some circumstances, be desirable to give a degree of curvature to the whole panel, for example to match the curvature of a face (such as a building facade) on which the panel is mounted. The curvature could be in the horizontal and/or the vertical direction.