Abstract:
A solar collector system is provided. The solar collector system comprises a frame member and a top plate supported by the frame member with the top plate being transparent to solar energy. A membrane is supported within the frame beneath the top plate and a solar absorber plate supported within the frame beneath the membrane. A collector or plurality of collectors removes heat from the solar absorber plate. The collectors have a fluid selectively flowable through the heat collectors wherein upon the membrane achieving a first predetermined temperature, the membrane becomes substantially taut within the frame and spaced from the solar absorber plate and wherein upon the membrane achieving a second predetermined temperature, the membrane becomes substantially flaccid and contacts the solar absorber plate thereby maintaining the solar absorber plate below the second predetermined temperature.

Description:
[0001]    The present application is a continuation of pending provisional patent application Ser. No. 60/207,999, filed on May 26, 2000, entitled “Double Glazed Solar Collector System for Inhibiting Freezing During No-Flow Conditions”. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a solar collector system having a transparent film with a high thermal expansion coefficient and, more particularly, it relates to a solar collector system, which inhibits excessive solar collector solar absorber plate temperatures during no-flow conditions.  
           [0004]    2. Description of the Prior Art  
           [0005]    In a conventional double-glazed solar collector system, when the flow of liquid ceases through the system, extreme component temperatures may be produced. High temperature materials are required for the absorber assembly and inner glazing of an efficient system to prevent physical harm to the structure thereby damaging the solar collector. Therefore, conventional solar collectors that utilize inexpensive but low temperature materials for the solar absorber plate (such as plastics) must be downgraded to limit their maximum operating temperature. This is often accomplished by using an ineffective single glazed system and/or a nonselective surface absorber.  
           [0006]    A need therefore exists in the art for a solar collector system, which inhibits excessive solar collector solar absorber plate temperatures during no-flow conditions. It is desirable that this be achieved, moreover, without compromising the solar collector system performance and the effectiveness of the materials. The present invention solves these problems and offers other advantages over the prior art.  
         SUMMARY  
         [0007]    The present invention is a solar collector system. The solar collector system comprises a frame member and a top plate supported by the frame member with the top plate being transparent to solar energy. A membrane is supported within the frame beneath the top plate and a solar absorber plate supported within the frame beneath the membrane. A collector or plurality of collectors removes heat from the solar absorber plate. The heat collectors have a fluid selectively flowable through the heat collectors wherein upon the membrane achieving a first predetermined temperature, the membrane becomes substantially taut within the frame and spaced from the solar absorber plate below the first predetermined temperature and wherein upon the membrane achieving a second predetermined temperature, the membrane becomes substantially flaccid and contacts the solar absorber plate thereby maintaining the solar absorber plate at the second predetermined temperature.  
           [0008]    The present invention additionally includes a system for collecting solar energy. The system has a solar absorber plate for absorbing solar energy and heat collectors for collecting the solar energy from the solar absorber plate during flow conditions. The system comprises means spaced from the solar absorber plate during flow conditions and contactable with the solar absorber plate during no-flow conditions for reducing the temperature of the solar absorber plate.  
           [0009]    The present invention further includes a solar collector having a double-glazed configuration. The solar collector comprises means for transforming from a double-glazed configuration into a single-glazed configuration upon occurrence of a predetermined event. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a sectional side view illustrating a solar collector system, constructed in accordance with the present invention, with the inner glazing film being in a taut condition during a fluid flow event; and  
         [0011]    [0011]FIG. 2 is a sectional side view illustrating the solar collector system, constructed in accordance with the present invention, with the inner glazing film being in a sagged, elevated temperature condition during a high ambient temperature and high solar radiation, fluid no-flow or stagnation event. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    As illustrated in FIGS. 1 and 2, a solar collector system, indicated generally at  10 , passively, reliably, and inexpensively transforms a double-glazed solar collector system into a single-glazed solar collector system when the absorber starts to overheat. This extends the range of possible absorber materials that can be used without decreasing performance. Actual construction and operation of the solar collector system  10  will be described in detail below.  
         [0013]    To accomplish the desired result, the solar collector system  10  has a supporting frame  12  having a first frame member  14  and a second frame member  16 . A top plate  18  is supported on the first frame member  14  and the second frame member  16  to inhibit damage of the internal mechanisms (as described below) of the solar collector system  10  from weather elements. The transparent top plate  18  can be constructed from any material, which is transparent to solar radiation. For instance, the top plate  18  is preferably constructed from a glass material although it is within the scope of the present invention to construct the top plate  18  from any solar transparent material including, but not limited to, Sunlite HP, Lucite acrylic, Tedlar PVF, etc.  
         [0014]    The solar collector system  10  of the present invention further includes a solar absorber plate  20  supported between the first frame member  14  and the second frame member  16  of the supporting frame  12 . The solar absorber plate  20  absorbs solar energy entering between the first frame member  14  and the second frame member  16  of the supporting frame  12  through the top plate  18 .  
         [0015]    Beneath the solar absorber plate  20  of the solar collector system  10  is a single or plurality of heat collectors  22  removing the heat of the solar absorber plate  20  caused by solar energy. The fluid travels through a pipe or channel collecting heat from the solar absorber plate  20  and cooling the solar absorber plate  20 . An insulation material  24  can surround the heat collectors  22  to minimize heat loss of the fluid traveling therethrough.  
         [0016]    Between the top plate  18  and the solar absorber plate  20 , an inner glazing film  26  is supported between the first frame member  14  and the second frame member  16  of the supporting frame  12 . The inner glazing film  26  is a mechanically resilient, transparent film, which can withstand many thermal cycles, have a large coefficient of thermal expansion and a high operating temperature. Additionally, the inner glazing film must not become affixed to the solar absorber plate  20 .  
         [0017]    Preferably, the inner glazing film  26  is a membrane  26  constructed from a Teflon CLP material although other types of membranes  26 , including, but not limited to, a Teflon PFA material, are within the scope of the present invention. The actual composition of the membrane  26  is preferably a transparent fluorinated carbon, which tends to be receptive to expansion with an increase in temperature.  
         [0018]    During normal operation of the solar collector system  10  of the present invention, the solar energy passes through the solar transparent top plate  18  and the inner glazing film  26 . The solar energy strikes the solar absorber plate  20  causing the temperature of the solar absorber plate  20  to be increased. The energy from this temperature increase is then transferred to the fluid, typically an antifreeze substance, flowing through the heat collectors  22  and through the glazing to the ambient air. While most of the absorbed thermal energy is transferred from the hot absorber plate to the colder fluid, some of the thermal energy is also transferred to the colder ambient air. Both of these transfer rates are proportional to the temperature difference between the mean solar absorber plate temperature and the respective fluid temperature.  
         [0019]    As illustrated in FIG. 1, under normal operating conditions, the inner glazing film or the membrane  26  is designed to remain taut between the first frame member  14  and the second frame member  16  of the supporting frame  12 . As illustrated in FIG. 2, when the fluid flow within the heat collectors  22  has ceased (stagnate or no-flow while still exposed to intense solar irradiation), the temperature of the all the components will increase, especially the solar absorber plate  20  and the components in its immediate vicinity. When the solar absorber plate  20  starts to overheat, the corresponding temperature increase of the inner glazing film  26  causes it to become flaccid and sag upon the solar absorber plate  20 . The sagging of the inner glazing film  26  causes the inner glazing film  26  to substantially contact the solar absorber plate  20  effectively eliminating the gap, i.e., the solar area  28  between the solar absorber plate  20  and the inner glazing film  26 . The elimination of the solar collection area  28  inhibits the minimizes the temperature difference between the solar absorber plate  20  and the inner glazing film  26  during no-flow conditions through the heat collectors  22 .  
         [0020]    With the elimination of the solar collection area  28 , the temperature of the membrane  26  is maintained substantially equivalent to the temperature of the absorber plate  20 . While these two temperatures are essentially equivalent, the solar absorber plate  20  is still dictating all the temperatures. For instance, there are two parallel paths for the absorbed solar energy as illustrated in FIG. 1, namely a low resistive heat transfer path from the solar absorber plate  20  to the fluid and a high resistive path from the solar absorber plate  20  to the ambient air. For a given temperature difference between the solar absorber plate  20  and the two fluid temperatures (fluid and ambient air), most of the energy is transported through the low resistive path (solar absorber plate  20  to fluid). During stagnation, only the high resistance path is available and it requires a large temperature difference between the solar absorber plate  20  (stagnation temperature) and the ambient air to transfer the same amount of absorbed solar energy to the air. The sagging of the inner glazing film  26  causes the magnitude of the solar absorber plate  20  to the ambient air resistance to significantly decrease (double glazing resistance to single glazing resistance) which translates into a large decrease in the required stagnation temperature to dissipate the absorbed solar energy.  
         [0021]    Basically, in the solar collector system  10  of the present invention, the membrane  26  inhibits damage to the solar absorber plate  20  by transforming the solar collector system  10  from a double-glazed collector system to a single-glazed collector system during no-flow conditions. For example, when the fluid flowing through the heat collectors  22  beneath the solar absorber plate  20  ceases, the temperature begins to build in the solar collection area  28  between the membrane and the solar absorber plate  20 . As the solar collection area  28  temperature increases, the temperature of the membrane  26  also increases in a corresponding manner. The molecules of the membrane  26  become more excited with the increased temperature and begin to increase the distance between the centers of the molecules causing the membrane  26  to become flaccid and drape closer to and upon the surface of the solar absorber plate  20  until substantially the entire membrane  26  is contacting the solar absorber plate  20 .  
         [0022]    Therefore, the solar energy entering the solar collector system  10  simply strikes the loose and flaccid inner glazing film  26  and the solar absorber plate  20 . This significantly increases the heat transfer conductance between the solar absorber plate  20  and the ambient air, which prevents the solar absorber plate  20  from overheating. The air may or may not circulate in cavity  30  that depend upon the temperature difference across the cavity  30  and the cube of its gap distance  30 . The temperatures of the solar absorber plate  20  and the inner glazing film  26  are not necessarily equal to the temperature of the cavity  30  whose temperature varies across the gap  30  from the temperature of  26  to the temperature of  18 . Once the circulation of the fluid through the heat collector  22  commences, the temperature in the solar collector system  10  will lower and the inner glazing film  26  move away from the solar absorber plate  20  and becomes taut thereby returning the solar collector system  10  to its normal operation.  
         [0023]    Experiments with the solar collector system  10  of the present invention have revealed that when the experimentally measured solar absorber plate  20  stagnation temperature was approximately one hundred and sixteen (116° C.) degrees Celsius, the outer glazing temperature was approximately fifty (50° C.) degrees Celsius. If the inner glazing film  26  were not properly engineered so that it had remained taut, the stagnation temperature of the solar absorber plate  20  would have an estimated temperature of approximately one hundred and sixty-six (166° C.) degrees Celsius. Therefore, the solar collection system  10  of the present invention passively decreased the stagnation temperature of the solar absorber plate  20  by approximately fifty (50° C.) degrees Celsius.  
         [0024]    The performance of solar collectors that utilize inexpensive, but low temperature materials, for the solar absorber plate  20  (such as plastics) must be downgraded to limit the solar collectors&#39; maximum operating temperature. In conventional solar collectors, this is accomplished by using a system with no glazing, a single glazing, and/or a non-selective radiation surface. The solar collector system  10  of the present invention is engineered to passively, reliably, and inexpensively transform a double glazed system to a single glazed system for elevated no-flow, stagnation temperatures thereby extending the range of possible absorber materials that can be used without decreasing performance.  
         [0025]    The foregoing exemplary descriptions and the illustrative preferred embodiments of the present invention have been explained in the drawings and described in detail, with varying modifications and alternative embodiments being taught. While the invention has been so shown, described and illustrated, it should be understood by those skilled in the art that equivalent changes in form and detail may be made therein without departing from the true spirit and scope of the invention, and that the scope of the present invention is to be limited only to the claims except as precluded by the prior art. Moreover, the invention as disclosed herein, may be suitably practiced in the absence of the specific elements, which are disclosed herein.