Abstract:
A solar heater has a light collector which collects light incident on the light collector into a beam that extends along a beam axis. A plurality of thermally conductive plates is arranged along the beam axis, each of the plates having an aperture on the beam axis. For each plate, the aperture of the plate is smaller than apertures of all plates disposed between each plate and the light collector along said beam axis. A fluid pathway is associated with one or more of the plates for transferring heat to a fluid in the fluid pathway. In a related method of heating a fluid, light rays are collected into a beam and passed through progressively smaller apertures in a plurality of serially arranged plates such that a portion of the beam is incident on each one of said plates. The fluid is thermally contacted with the plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 USC 119(e) from provisional application No. 61/537,728. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to solar heaters and to methods of solar heating. 
         [0003]    Given the environmental concerns arising from the use of fossil fuels and the environmental risks associated with the production of nuclear energy, there is an increasing interest in the use of “green” technologies for energy production. One green technology which has been subject of significant development is that of harnessing solar energy. There is therefore a continued need for improved approaches to harness solar energy. 
       SUMMARY 
       [0004]    A solar heater and a solar heating method utilize a plurality of plates with apertures that decrease in size in a direction of propagation of a light beam. 
         [0005]    According to an aspect, a solar heater has a light collector which collects light incident on the light collector into a beam that extends along a beam axis. A plurality of thermally conductive plates is arranged along the beam axis, each of the plates having an aperture on the beam axis. For each plate, the aperture of the plate is smaller than apertures of all plates disposed between each plate and the light collector along said beam axis. A fluid pathway is associated with one or more of the plates for transferring heat to a fluid in the fluid pathway. 
         [0006]    According to another aspect, in a method of heating a fluid, light rays are collected into a beam and passed through progressively smaller apertures in a plurality of serially arranged plates such that a portion of the beam is incident on each one of said plates. The fluid is thermally contacted with the plates. 
         [0007]    Other features and aspects of the invention will become apparent from the following description in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the figures which illustrate, by way of example only, embodiments of the present disclosure: 
           [0009]      FIG. 1  is a schematic view of a solar heating system made in accordance with an embodiment; 
           [0010]      FIG. 2  is a schematic view of a portion of the system of  FIG. 1 ; 
           [0011]      FIG. 3  a perspective view of a portion of the solar heating system of  FIG. 1 ; 
           [0012]      FIG. 4A  is an exploded view of a portion of  FIG. 2 ; 
           [0013]      FIG. 4B  is a schematic cross-sectional view of a portion of  FIG. 2 ; and 
           [0014]      FIG. 5  is an exploded view of a portion of a solar heating system made in accordance with another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    With reference to  FIG. 1 , an example solar heating system  10  for heating a working fluid has a solar heater  12  mounted through a positioning system  15  to support arm  16 . A fluid conduit  14  which is connected to a cold fluid outlet  18  of a storage tank  20 , runs to the solar heater  12 , and a second fluid conduit  17  exits the heater and terminates at a hot fluid inlet  22  of storage tank  20 . A back-up system  30  to solar heating system  10  is in heat exchange relationship with storage tank  20 . A hot fluid outlet  24  of tank  20  may connect to an external system (not shown), such as a domestic heating system. The storage tank holds a working fluid, such as water. Back-up system  30  could include a back-up boiler and a back-up burner, such as a conventional oil or gas fired or electric burner. 
         [0016]    A clevis  32  extending from the solar heater  12  is pivotably mounted at pivot  34  to base  36 . The base  36  is in turn mounted to support arm  16  through a first pivot (not shown) so as to rotate about the longitudinal axis of the support arm and a second pivot (not shown) so as to tilt in a direction which is both perpendicular to the support axis and perpendicular to the axis of the pivot  34 . Turning to  FIG. 2 , servo motor  38  of positioning system  15  acts to tilt the solar heater about pivot  34 ; servo motor  40  acts to tilt the solar heater about a pivot which is both perpendicular to the support axis and perpendicular to the axis of the pivot  34 ; and servo motor  42  acts to pivot the solar heater about the longitudinal axis of the support arm  16 . The servo motors input computer-controller  44 ; the controller  44  also receives an input signal from solar intensity sensor  46 . 
         [0017]    The controller  44  operates to control the servo motors of positioning system  15  so as to orient solar heater  12  to follow the path of the sun. Thus, controller  44  and positioning system  15  act as a solar tracking system. Since solar tracking systems are conventional, the solar tracking system is believed to be within the skill of one skilled in the art and is not further described. Optionally, a photovoltaic panel (not shown) may be used to provide power to the controller and the servo motors. 
         [0018]    As seen in  FIG. 1 , solar heater has a housing  26 .  FIG. 3  illustrates the solar heater  12  without this housing. With reference to  FIG. 3 , solar heater  12  includes a light collector in the nature of a focusing lens  50  supported by arms  52  at a stand-off from a heat exchanger  54 . When facing the sun, the lens  50  directs light along a beam axis coincident with a central axis C and focuses this light at a focal point at heat exchanger  54 . With reference to  FIG. 4A , the heat exchanger  54  has upper housing cups  56  and  58  and lower housing disc  60  encasing a stack of plates  64   a,    64   b,  and  64   c  (collectively, plates  64 ) extending transversely of, and aligned along, central axis C. The upper housing cups  56 ,  58  and each plate  64  has a central aperture, collectively, apertures  68 , centered on central axis C. The plates  64  are made of a thermally conductive material, such as metal, as, for example, aluminum. 
         [0019]    The central aperture in each of the upper housing cups  56 ,  58  is larger than the aperture in top plate  64   a.  As is most easily seen in  FIG. 4B , each apertured plate  64  has a different sized aperture and the plates are arranged so that the apertures progressively decrease in size in a direction away from lens  50 . Thus, plate  64   a , which is closest to lens  50 , has the largest aperture  68   a;  plate  64   b  next closest to lens  50  has the second largest aperture  68   b,  and plate  64   c  the smallest aperture  68   c.    
         [0020]    The underside of each plate  64  is etched to form a spiral pathway  70 ; when the plates  64  are sandwiched together as shown in  FIG. 4B , the underside of each spiral pathway is closed off. An opening  72  through the top of middle plate  64   b  communicates the outer end of the spiral pathway of the top plate with the outer end of the spiral pathway of the middle plate. An opening  74  through the top of the lower plate  64   c  communications the inner end of the spiral pathway of the middle plate  64   b  with the inner end of the spiral pathway of the lower plate  64   c.  Aligned openings in housing disc  60  and plates  64   b,    64   c  receive the end of conduit  17  which terminates at the inner end of the spiral pathway of the top plate  64   a.  A further opening in the housing disc  60  receives conduit  14  which terminates at the outer end of the spiral pathway of the bottom plate  64   c.    
         [0021]    It will be apparent from  FIG. 4B  that the stack of plates  64  extends a relatively short distance along the central axis C. Consequently, the lens  50  can have a focal plane somewhere within the heat exchanger and the light with be substantially in focus at each plate of the stack of plates. The characteristics of the lens are chosen such that sunlight focused by the lens  50  has, at each plate  64   a,    64   b,    64   c,  a beamwidth greater than the diameter of the aperture  68   a,    68   b,    68   c  through the plate. 
         [0022]    In use, solar heater  12  is located in an area where it is exposed to sunlight. Controller  44  may then control servo motors  38 ,  40 , and  42  in order to adjust the orientation of solar heater  12  through the day so that the lens  50  constantly faces the sun. 
         [0023]    Since the apertures  68  of each successive plate  64  in a downstream direction decrease in size, for any given apertured plate downstream of plate  64   a,  a peripheral portion of the part of the beam that passes through the aperture of the plate immediately upstream of the given plate will be incident on the given plate  64  while a central portion of the beam will pass through the aperture of this given plate. The portion of the beam passing through aperture  68   c  of the last apertured plate  64   c  is incident upon solid plate  60 . Thus, it will be apparent that a portion of the light from the beam is incident on each one of plates  64 . 
         [0024]    The portion of the beam striking a plate  64  transfers energy to the plate, and therefore heats the plate. While the beam strikes only the more central region of a plate, since the plates are made of a thermally conductive material, this heat will diffuse across the plate. 
         [0025]    As plates  64  are heated, working fluid within the spiral pathway  70  of each plate is heated. This reduces the density of the working fluid and will result in a convective flow through the conduit  14  from cold fluid outlet  18  of tank  20  to the hot fluid inlet  22  of the tank, given a judicious choice for the height of the cold fluid outlet  18  relative to the plates  64 . Flow may be more readily assured if, as shown, the conduit  14  is connected so that fluid flowing from the cold fluid outlet  18  flows first through the spiral path  70  of the bottom plate  64   c.  Alternatively, and especially if the conduit is connected so that fluid flowing from the cold fluid outlet  18  flows first through the spiral pathway  70  of the top plate  64   c,  a pump (not shown) may be placed in fluid conduit  14  to pump fluid through the conduit  14  to hot fluid inlet  22 . 
         [0026]    The temperature at which working fluid is discharged from fluid conduit  14  depends on the quantity of solar energy collected by solar heater  12 , the efficiency of heat transfer to the working fluid, and the flow rate of the working fluid, among other factors. The quantity of solar energy collected by solar heater  12  is directly proportional to the area of lens  50  and the intensity of the incident solar radiation. 
         [0027]    Heated working fluid in tank  20  may be drawn off for use at the tank&#39;s hot fluid outlet  24 . 
         [0028]    In some circumstances, it may be desirable to supplement the supply of heated working fluid from the solar heater  12 . For example, on a cloudy day, solar heater  12  may not heat the working fluid to the desired temperature. If this occurs, additional heating may be provided by back-up system  30 . Back-up system  30  may be operated in parallel with solar heater  12 , supplementing its output. The back-up system may also be operated when solar heater  12  inactive, such as at night. 
         [0029]    The heated fluid produced by solar heating system  10  can be used for a variety of purposes. For example, heated fluid may be circulated through pipes and radiators in a building to provide heating for the building. Also, if the working fluid is water, solar heating system  10  may be used to provide a supply of warm or hot water for domestic use. 
         [0030]    Optionally, lens  50  could be replaced with a different light collector, such as a reflector. 
         [0031]    In another embodiment, with reference to  FIG. 4 , the solar heater  112  includes a collector  160 , a collimator  162 , and a heat exchanger  154  with a stack of plates  164   a,    164   b,    164   c,    164   d,  and  164   e  (collectively, plates  164 ) extending transversely of, and aligned along, a central axis C. Each plate, except plate  164   e , has a central aperture  168   a,    168   b,    168   c,  and  168   d  (collectively, apertures  168 ) centered on central axis C. In the example embodiment, collector  160  and collimator  162  are lenses positioned so that the collector lens has a focal point, F, located between the collector lens and the collimator lens and so that focal point F is also a focal point of the collimator lens. The heating power of solar heater  112  is proportional to the amount of solar radiation incident on collector  160 , i.e., to the area of collector  160 . Accordingly, collector  160  may be as large as available space permits. 
         [0032]    Each apertured plate  164  has a different sized aperture and the plates are arranged so that the apertures progressively decrease in size in a direction away from collimator  162 . Thus, plate  164   a,  which is closest to collimator  162 , has the largest aperture; plate  164   b  next closest to collimator  162  has the second largest aperture, plate  164   c  the next largest aperture, and plate  164   d  the smallest aperture. 
         [0033]    Fluid conduit  14  terminates at a coil  169  with a looped portion  170  lying on each plate  164 ; each looped portion may be soldered, or otherwise affixed, to the underlying plate. The coil, or at least each looped portion  170  of the coil, is formed of a thermally conductive material so as to permit heat transfer from the plates  164 . 
         [0034]    In use, collector lens  160  concentrates light by focusing it at focal point F and the light then diverges from this point and is collimated by the collimator lens. Collimator  162  collimates light into a beam having a beam axis coincident with central axis C. Collimator  162  is configured so that this beam has a diameter greater than that of the aperture  168   a  in the topmost plate  164   a,  such that a peripheral portion of the beam is incident on plate  164   a  while the central portion of the beam passes through aperture  168   a.    
         [0035]    Since the aperture  168  of each successive plate in a downstream direction decreases in size, for any given apertured plate downstream of plate  164   a,  a peripheral portion of the part of the beam that passes through the aperture of the plate immediately upstream of the given plate will be incident on the given plate  164  while a central portion of the beam will pass through the aperture of this given plate. The portion of the beam passing through aperture  168   d  of the last apertured plate  164   d  is incident upon solid plate  164   e.  Thus, it will be apparent that a portion of the light from the beam is incident on each one of plates  164 . 
         [0036]    The portion of the beam striking a plate  164  transfers energy to the plate, and therefore heats the plate. While the beam strikes only the more central region of a plate, since the plates are made of a thermally conductive material, this heat will diffuse across the plate and heat the working fluid in the loops  170  of coil  169 . 
         [0037]    In an alternate embodiment, the collecting lens could be replaced with a parabolic reflector disposed below a stack of plates and reflecting light back up through a lens-type collimator and through the stack of plates. With such an embodiment, the plate with the largest aperture would be at the bottom of the stack. In a further alternate embodiment, collimator  162  could be a reflector. 
         [0038]    While in the illustrated embodiment the loops  170  of the coil lie on each plate, they could obviously instead be associated with the underside of each plate. While the loops  170  are depicted near the periphery of each plate, they could instead be located near the center of each plate, where the plate temperature is highest. In another embodiment, there could be multiple loops  170  on each plate. 
         [0039]    For a given collector size, the quantity of solar energy transferred to a particular plate  164  is proportional to the size of the annular portion of the plate which is exposed to the beam. The relative sizes of successive apertures  168  can therefore be adjusted to control the peak temperature of each plate during operation in order to maximize the heat transfer to working fluid in the conduit  14 . 
         [0040]    While in the illustrated example embodiment of  FIGS. 1 to 4  there are three plates and in the illustrated embodiment of  FIG. 5  there are five plates, obviously a different number of plates may be used. Where, for example, the  FIG. 5  embodiment were modified so that there are eleven plates, the plate closest to the collimator could have an aperture with a diameter of 1.1″ and each successive plate could have an aperture 0.1″ smaller such that the tenth plate has an aperture with a diameter of 0.2″. As with the embodiment of  FIG. 5 , the eleventh plate could be a solid plate (lacking an aperture). 
         [0041]    While the apertures of the illustrated embodiments are circular, they could also have other shapes. Further, the apertures of the plates do not need to have identical shapes provided they are progressively smaller so as to permit a portion of the beam to strike each plate. 
         [0042]    While the illustrated embodiment shows a single tank, optionally, there could be a cold fluid reservoir and a separate hot fluid reservoir. Moreover, if the working fluid is water, the cold fluid reservoir may be a naturally occurring reservoir such as a lake or other body of water. As a further option, there may be no hot fluid reservoir and instead heated fluid may simply be discharged to where it is to be used. 
         [0043]    The solar tracking system of the example embodiment may be replaced with any suitable solar tracking system, Indeed, while not as efficient, for some applications, solar heater may be used without a solar tracking system. 
         [0044]    An advantage of the present solar heating system is its compact horizontal extent, allowing the solar heater  12  or  112  to be placed in a number of constrained spaces. 
         [0045]    Other modifications will be apparent to those skilled in the art; therefore, the invention is defined in the claims.