Abstract:
A high-efficiency solar radiation collection and conversion system is described. An array of evacuated collector tubes each includes two or more inner heat pipes that capture the solar energy and conduct it as heat through a condenser portion into a manifold that operates as part of a closed-loop circulation system. In another part of the loop, a heat exchanger transfers the heat into a hot-water holding tank or otherwise applies the heat energy in the circulating fluid.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a nonprovisional of, and claims priority to, U.S. Provisional Application No. 61/305,135, filed Feb. 16, 2010, with title “High Efficiency Conversion of Solar Radiation into Thermal Energy,” pending. The entire disclosure in that application is incorporated herein by reference as if fully set forth. 
     
    
     FIELD 
       [0002]    This disclosure generally relates to the use of solar heat. More particularly, the disclosure relates to solar heat collectors having working fluid conveyed through the collector and having means to exchange heat between plural fluids. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  is a schematic view of a solar radiation conversion system according to one embodiment. 
           [0004]      FIG. 2  is a top section view of a header and array of solar collection tubes for use in the embodiment of  FIG. 1 . 
           [0005]      FIG. 3  is a magnified section view of a header and part of an array of collection tubes used in the embodiment of  FIG. 1 . 
       
    
    
     DESCRIPTION 
       [0006]    For the purpose of promoting an understanding of the principles of the invention, reference will now be made to certain embodiments illustrated in the disclosure, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
         [0007]    Generally, with reference to  FIG. 1 , system  100  includes a solar collection subsystem  110 , a closed-loop circulation system  115 , and tank  130 . Solar collection subsystem  110  receives incident solar radiation E and moves the heat energy into the fluid flowing through circulation system  115 . That fluid circulates through tank  130 , where heat is drawn out, and back to solar collection subsystem  110 . 
         [0008]    In particular, solar collection subsystem  110  includes collection tubes  112  (described further herein), header assembly  116 , support members  114 , and one or more legs  118  that support the other components of solar collection subsystem  110  at a desired angle and in a desired position. Each collection tube  112  includes a double-wall outer tube (a substantially transparent outer cylinder and an inner cylinder adapted to pass light and hold heat, fused together at the ends with evacuated space in between) that contains two or more inner “heat pipes,” which carry the heat energy up to the highest point in the tube. There the heat is transferred to recirculating fluid  128  (see  FIG. 2 ) in header assembly  116 . Pump  120  moves recirculating fluid  128  through closed-loop circulation subsystem  115 , which includes pipe  122 , header  217  (see  FIGS. 2-3 ) in header assembly  116 , pipe  124 , tank  130 , and pipe  126 . Within tank  130 , heat exchanger  132  pulls heat out of recirculating fluid  128  and into the water in tank  130 . The heated water in the storage tank  130  is then available for many uses, as will occur to those skilled in the art, including without limitation domestic hot water, heated water for radiant floor heating, recovery water for boiler systems, commercial hot water systems, and other applications. 
         [0009]    In other embodiments, tank  130  holds fluid other than water, which is likewise used as will occur to those skilled in the art. In still other embodiments, recirculating fluid  128  transports heat energy to any other load for using heat energy that will occur to those skilled in the art. In these various systems, recirculating fluid  128  may be a mixture of  70 % propylene glycol and 30% water, or it may be any other fluid suitable for heat transport as will occur to those skilled in the art. 
         [0010]      FIG. 2  illustrates an array  210  of collection tubes  212  that cooperate to capture solar energy for the system. As discussed elsewhere herein, each collection tube  212  connects with manifold  217  (within manifold assembly  214 ), where recirculating fluid  128  captures the heat. Recirculating fluid  128  enters manifold  217  through inlet  216  and exits through outlet  218 , flowing through the rest of circulation subsystem  115  as discussed above. End caps  221  protect the ends of collection tubes  212  and hold collection tubes  212  in position. They may be made from plastic, metal, or other material as will occur to those skilled in the art. 
         [0011]      FIG. 3  illustrates more detail about collection tubes  212  and their interface with manifold  217 . In this embodiment, each collection tube  212  is a double-wall glass tube made of a transparent outer cylinder and an inner cylinder coated with a selective coating (such as AIN/AI) that features excellent solar radiation absorption and minimal reflection properties. The ends  213  of the cylinders are fused together as the space between them is evacuated at high temperature in order to create and maintain a vacuum gap between the cylinders. 
         [0012]    The transparency of the outer cylinder allows light rays to pass through with minimal reflection. The inner cylinder absorbs radiation and reflects only minimal amounts thereof. The evacuated space between the inner and outer cylinders helps the efficiency of the collection subsystem in several ways, including but not limited to reducing the amount of radiant energy that is absorbed by matter in that evacuated space  224 ; reducing the overall mass of the system; and avoiding losses due to conduction of heat from the heat pipes  220  to the ambient air  226 . 
         [0013]    Within each collection tube  212 , in the space inside the inner cylinder, are two or more heat pipes  220 . Each heat pipe  220  in this embodiment is made of high-purity copper, containing only trace amounts of oxygen and other elements. These and other implementations of the invention will have different and additional advantages as will occur to those skilled in the art. 
         [0014]    In operation, heat pipes  220  function to capture incident radiant energy as heat and transfer that heat to header  217 . Each heat pipe  220  is evacuated, and a small quantity of purified water and/or other fluid (as will occur to those skilled in the art) is added. By evacuating the heat pipes  220 , one lowers the temperature at which the fluid evaporates in the tube. In one embodiment, the heat pipes  220  have a boiling point of only 30° C. (86° F.), so when the heat pipe  220  is heated above that temperature, the fluid vaporizes. This vapor rapidly rises to the condenser  222  located at the top of the heat pipe  220 . This condenser is inserted into header pipe  217 . A mixture  228  of 70% propylene glycol and 30% water is pumped through the header  214 , absorbing via condenser  222  the thermal energy harvested by the heat pipe  220 . As this heat is drawn from the condenser, the vapor in inner tube  220  condenses in condenser  222  and returns to the bottom of the heat pipe  220  to repeat the process. 
         [0015]    Even though heat pipe  220  is evacuated and the boiling point of the fluid inside has been reduced, the freezing point of that fluid is still the same as at sea level (which, in this embodiment, is 0° C. (32° F.)). Because the heat pipe  220  is located within the inner cylinder, protected from losses to ambient air  226  by the vacuum gap  224 , brief overnight temperatures as low as −20° C. (14° F.) will not cause the heat pipes  220  to freeze. Plain water heat pipes may be damaged by repeated freezing. The water used in the heat pipes in the present system still freezes in cold conditions, but it freezes in a controlled way that does not cause swelling of or damage to the copper pipe. 
         [0016]    The use of two or more heat pipes  220  within each collection tube  212  provides additional advantage over other designs. For example, having two or more heat pipes within each solar collection tube provides significantly greater density in the overall collection subsystem than other designs. Further, this aspect of the present design is complimentary to other techniques for improving capture of solar radiation in solar collection systems, and can be combined with techniques like using lenses or reflectors to concentrate the solar radiation before it is captured. Other radiation concentration techniques can be used with this system as will occur to those skilled in the art in view of this disclosure. 
         [0017]    In various embodiments, two, three, four, or more heat pipes may be contained within each outer tube and connected to the closed circulation path via conductive heat transfer. In other embodiments, multiple collection manifolds receive heat from the condenser portions of the heat pipes, running (as a non-limiting example) in parallel through the manifold enclosure. 
         [0018]    While the inventions have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that the preferred embodiment has been shown and described and that changes and modifications that come within the spirit of the invention are desired to be protected.