Abstract:
Provided is a heat collector comprising a linear tubular heat collector member defining an internal passage for conducting fluid; a base member provided with a front surface defining a trough-shaped recess partly surrounding the heat collector member; and a plate member fitted in the recess and provided with a reflective front surface. Preferably, the plate member is engaged by the base member at both side edges thereof, and placed under a compressive load so as to undergo a buckling deformation and substantially conform to a surface contour of the recess.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a radiation heat collector typically employed to collect solar heat to fluid media such as water, and in particular to a radiation heat collector including an improved reflector assembly and a method for making the same.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventionally, various forms of solar heat collectors have been proposed. Typically, a solar heat collector comprises a tubular heat collector member for conducting water that is to be heated, and a reflector for converging incident solar light upon the heat collector member. The reflector may consist of sheet metal having a reflective surface which is formed into a per se known CPC (compound parabolic concentrator) shape and reinforced by a plurality of ribs. The CPC is given as a combination of a pair of parabolic segments at an angle which is joined by an involute curve. The CPC allows an incident solar ray to efficiently converge upon the heat collector member placed in a bottom part thereof over a wide range of incident angle. For details of the CPC, reference should be made to U.S. Pat. No. 4,002,499 issued Jan. 11, 1977 to R. Winston.  
           [0003]    However, because the CPC surface has a relatively complex profile, it was difficult to form sheet metal into the desired shape, and the material for the reflector was required to have a significant thickness to ensure an adequate rigidity and maintain the original shape. Also, because of the spring back, it is difficult to give a prescribed shape to the stamp-formed sheet metal. Therefore, the manufacturing cost was high, and the reflector was undesirably heavy. Because such a solar heat collector is a consumer item, and typically placed on a roof, such disadvantages are highly detrimental for the market acceptability.  
           [0004]    Furthermore, the thermal efficiency of a solar heat collector is highly crucial in making the solar heat collector acceptable in the market because it has to compete with other forms of heat sources such as gas and electricity. Heat insulation for the collected heat is an important factor in determining the thermal efficiency, but again it has to be accomplished without substantially increasing the cost and weight of the solar heat collector.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    In view of such problems of the prior art and recognitions by the inventors, a primary object of the present invention is to provide a radiation heat collector which is easy to manufacture.  
           [0006]    A second object of the present invention is to provide a radiation heat collector which provides a high efficiency.  
           [0007]    A third object of the present invention is to provide a radiation heat collector which is economical to manufacture.  
           [0008]    A fourth object of the present invention is to provide a radiation heat collector which is light in weight and mechanically stable.  
           [0009]    According to the present invention, such objects can be accomplished by providing a radiation heat collector, comprising: a linear tubular heat collector member defining an internal passage for conducting fluid; a base member provided with a front surface defining a trough-shaped recess partly surrounding the heat collector member; and a plate member fitted in the recess and provided with a reflective front surface.  
           [0010]    Thus, by appropriately forming the shape of the recess of the base member which may consist of material such as heat insulating plastic material and can be therefore easily worked into a desired shape, the plate member can be formed into the desired shape without any substantial cost or difficulty. In particular, the plate member may be engaged by the base member at both side edges thereof, and placed under a compressive load so as to undergo a buckling deformation and substantially conform to a surface contour of the recess. For this purpose, a projection may be provided at least on one lateral side end of the recess for engaging an edge of the plate member.  
           [0011]    To facilitate the process of engaging the plate member with such a projection or projections, a gap may be defined between the reflective plate member and an opposing surface of the recess. Such a gap also contributes to the improvement of thermal insulation or thermal efficiency of the heat collector. Such gaps can be readily formed by forming localized recesses, localized projections or ridge-like projections which extend in a lengthwise direction on the surface of the trough-shaped recess.  
           [0012]    The reflective surface of the plate member preferably consists of a CPC reflective surface, and a central ridge may be provided in the plate member so as to form a CPC surface with a minimum amount of work and cost. Such a reflector arrangement can be achieved by forming a central ridge in the reflective plate member, typically by stamp forming, and appropriately fitting the plate member in the trough-shaped recess of the base member.  
           [0013]    The present invention also provides a method for manufacturing a radiation heat collector, comprising the steps of: preparing a base member having a trough-shaped recess on a front side thereof; providing an engagement portion on each lateral side end of the recess; and preparing a reflective plate member having a prescribed width, and engaging each side edge with a corresponding one of the engagement portions in such a manner that the plate member is placed under a compressive load and thereby undergo a buckling deformation until the plate member substantially conforms to a surface contour of the recess. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Now the present invention is described in the following with reference to the appended drawings, in which:  
         [0015]    [0015]FIG. 1 is a perspective view of a heat collector embodying the present invention;  
         [0016]    [0016]FIG. 2 is a cross sectional view showing one of the heat collector modules;  
         [0017]    [0017]FIG. 3 is an exploded cross sectional view showing the assembling process; and  
         [0018]    [0018]FIG. 4 is a view similar to FIG. 3 showing a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    [0019]FIG. 1 generally illustrates a radiation heat collector in the form of a solar water heater embodying the present invention. This heat collector comprises a casing  1  in the form of a box having an open top end, and a plurality of heat collector modules  2  placed one next to another inside the casing  1 . Each heat collector module  2  comprises a heat collector member  3  consisting of a tubular member, and a reflector  4  for reflecting solar light onto the heat collector member  3 . The tubular heat collector members  3  are connected to one another in series via a connecting pipe  3   a,  and one end of the serial connection is connected to a cool water source via a pump or the like while the other end thereof is connected to a user of hot water such as a storage tank and a radiator. If desired, the tubular heat collector members  3  may also be connected to one another in parallel. It is also possible to combine serial and parallel connections as desired.  
         [0020]    The open end of the casing  1  may be closed by a sheet or a plurality of sheets of glass or other transparent material  5  to thermally insulate the interior of the casing  1 . It has been proven to be advantageous to use a double glass pane, optionally having an evacuated intermediate layer. If desired, the interior of the casing  1  may be evacuated for an improved thermal insulation.  
         [0021]    Referring to FIG. 2, each reflector  4  comprises a base member  41  formed by molding plastic material such as foamed urethane or the like having a high level of thermal insulation and provided with a trough-like recess  41   a  extending along the length of the base member  41  on a front side thereof, and a reflector plate member  42  provided with a mirror surface on a front side thereof and attached to the recessed front side of the base member  41 . The reflector plate member  42  is typically made of sheet metal, but may also consist of a plastic sheet provided with a plated or otherwise formed mirror surface on a front side thereof.  
         [0022]    The cross sectional shape of the recess  41   a  is determined in such a manner that the reflective plate member  42  when fitted in the recess  41   a  presents a desired CPC surface which is defined by a pair of symmetric parabolic curves (A) opposing each other at a certain angle, and an involute curve (B) joining these two parabolic curves (A). A central ridge  42   b  is formed along the central part of the reflective plate member  42 . The central ridge  42   b  should touch the heat collector member  3  according to the CPC theory, but is spaced away from the heat collector member  3  in this embodiment to avoid the heat loss due to thermal conduction from the heat collector member  3  to the reflective plate member  42 .  
         [0023]    An engagement portion  41   b  consisting of a projection is provided on either side end of the trough-shaped recess  41 , and engages a corresponding edge of the reflective plate member  42 . A gap  41   c  is formed between the central ridge  42   b  of the reflective plate member  42  and the opposing inner surface of the recess  41 , and an additional gap  41   d  is formed between the reflective plate member  42  and the opposing inner surface of the recess  41   d  adjacent to either side end of the reflective plate member  42 . The gap  41   c  is defined by a central ridge  42   b  of the reflective plate member  42  as described in the following, and the gaps  41   d  are formed by locally recessing the opposing surface of the trough-shaped recess  41   a.    
         [0024]    If desired, the open end of the recess  41   a  of each heat collector  2  may be closed with a glass, plastic or other transparent sheet to improve heat insulation. A single heat collector  2  may be used individually, or a number of heat collectors  2  may be used as a group in any parallel and/or serial arrangement.  
         [0025]    The assembling process of the heat collector  2  is described in the following with reference to FIG. 3. The base member  41  provided with the recess  41   a  is formed by molding foamed urethane or other heat insulating material. This can be accomplished by using suitable mold dies. If desired, the base member  41  may be formed as a hollow member.  
         [0026]    The reflective plate member  42  is stamp formed by using suitable stamping dies, and is formed with a central ridge  42   b  extending along the lengthwise direction thereof. The reflective plate member  42  is dimensioned or trimmed so as to have a prescribed width so that when the reflective plate member  42  is pushed into the recess  41   a  and the two side edges are engaged by the engagement portions  41   b , the reflective plate member  42  takes a prescribed shape under its own resiliency. The gap  41   d  adjacent to each engagement portion  41   d  assists the process of engaging the corresponding edge with the corresponding engagement portion  41   b  by appropriately accommodating the deformation of the plate member  42  that is required or desired for executing this process. These gaps  41   d  as well as the gap  41   c  under the central ridge  42   b  also contribute to the improvement in thermal insulation by reducing the contact area between the reflective plate member  42  and the base member  41 . These gaps  41   c  and  41   d  additionally provide the function of accommodating the thermal expansion of the reflective plate member  42  with respect to the base member  41 .  
         [0027]    The gaps between the reflective plate and base member may be provided at will. In the embodiment illustrated in FIG. 4, the gap on each side end of the reflective plate member  42  is omitted, and a plurality of ribs  41   e  extending in the lengthwise direction are provided in the surface of the recess  41   a  of the base member  41  on either side of the central ridge  41   c . These ribs  41   e  not only create gaps between the reflective plate member  42  and base member  41  but also resiliently support the reflective plate member. These factors favorably accommodate any deformation of the reflective plate member which may be caused by thermal expansion, manufacturing error and other reasons, and facilitate the process of fitting the reflective plate member in place.  
         [0028]    Thus, according to the present invention, the radiation heat collector can be manufactured both easily and economically. Yet, the thermal efficiency and mechanical stability can be ensured at the same time.  
         [0029]    Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.