Abstract:
The invention relates to a field of open-flow solar collectors, and specifically to flat solar collectors with wetting the underneath sides of their solar radiation absorbing plates with liquid heat transfer medium. More specifically, the invention proposes the flat solar collector, which operates with relatively low flow rate of the heat transfer medium on the backside of the solar radiation absorbing plate, with flow in the form of some rivulets. The invention discloses some technical solutions, which restrict meandering rivulets&#39; flow. 
     These technical solutions are based on application of longitudinal strips attached by permanent magnets to the backside of the solar radiation absorbing plate fabricated from ferromagnetic metal.

Description:
[0001]    This application is a divisional of Ser. No. 13/714,697 filed Dec. 14, 2012, which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to the field of flat-plate solar collectors and, particularly, to open-flow flat-plate solar collectors. 
       BACKGROUND OF THE INVENTION 
       [0003]    The invention relates to a field of open-flow flat-plate solar collectors, and, specifically, to flat-plate solar collectors with wetting the underneath sides of their solar radiation absorbing plates by liquid heat transfer medium. 
         [0004]    Such solar flat-plate collectors and heat transferring units are disclosed in some US patents. 
         [0005]    U.S. Pat. No. 4,003,365 discloses a structure for collecting solar energy and storing the same as heat in a body of water. An inclined southerly wall is provided with a solar absorption surface protected from convection losses by a transparent membrane overlaying the absorption surface. The absorption surface is also formed as a membrane which is wetted at its underside within the structure by water spray and as solar radiation is absorbed and converted to heat at the membrane, the water at the underside of the membrane is heated. This heated water drops from the membrane and flows to a reservoir. 
         [0006]    U.S. Pat. No. 4,085,732 describes a method and apparatus for heating a liquid using solar energy. Through the use of an absorber plate made of a sheet of copper or any other similar conductive metal, the heat from the sun is captured. The front surface of the plate is covered with a dark absorbent coating. The heat absorbed by the copper sheet is conducted to a thin film of liquid such as water which is fed by gravity down the backside of the plate from a supply line disposed adjacent to the top of the backside of the plate. The liquid absorbs the heat from the plate as it flows downward covering the backside of the plate and is discharged at the bottom of the plate into a drain line. The drain line removes the heated liquid from the apparatus. 
         [0007]    U.S. Pat. No. 5,460,164 discloses a solar heat collector roof comprises an absorber plate ( 1 ) for incident radiation energy and a heat exchange plate placed directly thereunder. The heat exchange plate is corrugated in such a manner that it comprises channels extending down along the roof. The heat exchange plate is moreover in direct contact with the absorber plate thereabove. A heat carrying or heat removing liquid medium, e.g. water, flows in the channels. The liquid medium, which removes heat from the heat exchange plate, is supplied at such a rate that the flow rate down along the roof in the channels lies below the rate at which the surface tension of the heat carrying medium is broken, so that a winding (meandering) and travelling flow pattern is imparted thereby to the liquid medium, and up along the channel walls. 
         [0008]    U.S. Pat. No. 4,261,337 describes a solar heat collector includes an enclosure having a solar energy transmitting cover member and a solar energy absorbing base member. Within the enclosure, structure is provided, connected to a fluid inlet, for causing a stream or film of fluid to flow along the interior surface of the cover member. The same structure may be used to impart a separate flow of fluid along the energy absorbing base member. The cover member is oriented to prevent the gravitational forces on the fluid stream, flowing along the interior surface of the cover member, from overcoming the adhesion forces between the stream and the interior surface of the cover member. This stream removes condensation from the cover member as it captures heat therefrom. The stream may be created by spraying fluid on the interior surface of the cover member or by a fluid dispensing member having a fluid outlet adjacent the interior surface of the cover. Means for distributing the fluid across the interior surface as same flows therealong may also be provided. 
         [0009]    U.S. Pat. No. 4,121,568 teaches a system for collecting heat energy from solar radiation to heat a liquid medium. The system includes a solar radiation collector plate which has its rear face adapted to cause a liquid medium to be in direct thermal contact with the rear face of the plate while flowing over and covering essentially all of the rear face. According to one important aspect of the invention, the rear face is provided with a material having capillary attraction properties to spread a liquid medium across essentially all of the rear face of the plate so that the liquid medium absorbs large amounts of heat energy from the collector plate. 
         [0010]    U.S. Pat. No. 4,108,057 teaches a solar water heater is positioned in an inclined manner with an inlet at the upper portion and an outlet at the lower portion so that the water flows downwardly over a series of corrugations extending transversely to the direction of flow of water, the water spilling over each corrugation in turn. 
         [0011]    U.S. Pat. No. 4,124,020 describes a solar energy collector has a corrugated, inclined plate exposed to solar rays on a blackened front or top side thereof. A heat-absorbing liquid carrier adheres to an opposite rear or undersurface of the plate in the form of a thin sheet by surface tension, and is directed gravitationally to a collection trough at the bottom edge of the plate. 
         [0012]    U.S. Pat. No. 3,995,804 discloses an inclined heat absorptive and conductive panel including downwardly opening inverted V-shaped grooves formed therein extending downwardly from the upper end portion of the panel toward the lower end portion of the panel is provided. In addition, structure is provided for introducing a heat absorptive liquid into the upper end portions of the grooves and second structure is provided for receiving and collecting liquid from the lower ends of the grooves. The cross sectional shape and size of the grooves is such to allow at least substantially all of the liquid introduced into the upper ends thereof to be retained therein by the cohesive and surface tension properties of the liquid during its movement downwardly through the grooves by gravity toward the lower end of the panel. The panel comprises an inclined partition secured across the interior of an upwardly opening housing and a substantially fluid tight cover is secured across the top of the housing above the panel. Further, the structure by which liquid flowing downwardly to the lower end of the panel is collected includes additional structure whereby a partial vacuum is maintained within the housing between the transparent cover and the heat absorptive panel. 
         [0013]    U.S. Pat. No. 3,943,911 discloses a solar heat exchanger, which comprises: A. a base and an extended surface thereon for facing frontwardly toward the sun, B. means communicating with said surface to conduct liquid to flow in dispersed condition adjacent said surface, and C. a sheet overlying said surface and spaced therefrom sufficiently closely to cooperate therewith for filming the flowing liquid, said sheet adapted to receive solar radiation for promoting heat transfer to the filmed and flowing liquid 
         [0014]    U.S. Pat. No. 3,146,774 describes a solar collector, which is constructed similarly to the solar collector of U.S. Pat. No. 3,943,911. 
         [0015]    These patents have a common drawback: in order to ensure complete wetting of the backside of the absorbing plate they would be forced to apply relatively high flow rates (200 kg/mh or more) of water or aqueous solution supplied into their distributing pipes. This value of the required flow rate is presented, for example, in the book: S. S. Kutateladze HANDBOOK OF HYDRODYNAMIC PRESSURE DROPS AND HEAT TRANSFER, Energoatomizdat, Moscow 1990, p. 178 (in Russian). 
         [0016]    On the other hand, flow rate of 40 kg/mh or less is sufficient for a common 1.8-square-meter flat plate solar collector. For the flat plate solar collectors, which are intended to concentrate a diluted solution of liquid desiccant (as, for example, aqueous solutions of LiCl or CaCl.sub.2) this flow rate can be estimated as 10-15 l/hm. 
         [0017]    In such a way, the aforementioned common solar collectors require usage of pumps with relatively high power; it leads to additional expenses for equipment and electric energy. 
         [0018]    Only U.S. Pat. No. 3,995,804 has not this drawback and can operate with low flow rates. However, this patent does not give solution of anticorrosive polymer coating of the backside of the solar radiation absorbing plate (such polymer coating can fulfill the inverted V-shaped grooves described in this patent). In addition, U.S. Pat. No. 3,995,804 does not solve a problem of a relatively small general surface of rivulets flowing on the backside of the solar radiation absorbing panel. 
         [0019]    Secondly, it is known that oxygen entering into an open loop hydraulic solar system will cause rust in any iron or steel component. Such systems should have copper, bronze, brass, stainless steel, plastic, rubber components in the plumbing loop. 
         [0020]    Therefore, if the solar radiation absorbing plate is fabricated from a common carbon steel sheet, its backside to be coated with a layer of thermo-stable polymer anticorrosive material. 
       BRIEF SUMMARY OF THE INVENTION 
       [0021]    This invention proposes a design of a flat-plate solar collector, which is characterized by rivulets&#39; flow on the backside of its solar radiation absorbing plate; i.e. flow rate of water, antifreeze liquid medium or aqueous solution supplied on the upper section of the backside of the solar radiation absorbing plate is significantly lower than the minimum flow rate, which ensures formation of an entire liquid film flowing on the backside of this solar radiation absorbing plate. 
         [0022]    It is known that for low magnitudes of liquid flow rate on an inclined or vertical plate the liquid flow pattern is characterized by a system of narrow rivulets with relatively small width (for water and aqueous solutions in the order of 1-8 millimeters). 
         [0023]    Detailed theoretic analysis of rivulets&#39; flow and their stability is presented in the article: E. S. Benilov, “On the stability of shallow rivulets”, J. Fluid Mech. (2009), vol. 636, pp. 455÷474. Pp. 461÷462 of this article gives demonstration of stability of a rivulet flowing on the underside (backside) of an inclined plate. 
         [0024]    The article: A. Daerr et al. “General Mechanism for the Meandering Instability of Rivulets of Newtonian Fluids”, PHYSICAL REVIEW LETTER, SPRL 106, 184501 (2011) demonstrates that a rivulet flowing down an inclined plane often does not follow a straight path, but starts to meander spontaneously. This instability is the result of two key ingredients: fluid inertia and anisotropy of the friction between the rivulet and a substrate. Meandering only occurs if the motion normal to the instantaneous flow direction is more difficult than parallel to it. Above the threshold, the rivulet follows an irregular pattern with a typical wavelength of a few cm. 
         [0025]    The article: Nolwenn L E GRAND-PITEIRA et al. “What governs rivulet meanders on an inclined plane?”, Oct. 11, 2005, CCSD—00011140, Internet, shows that a rivulet flow is highly hysteretic: the shape of the meanders varies with flow rate only for increasing flow rates, and the straight rivulet regime does not appear for decreasing flow rate. 
         [0026]    Also, a main object of this invention is to provide simple means limiting the meandering phenomena of rivulets flowing on the backside of a solar radiation absorbing plate. 
         [0027]    A flat-plate solar collector, which is proposed in this invention, is designed from following main units: 
         [0028]    a housing with an internal thermal insulation of its bottom and side walls; the internal surfaces of the layers of the thermal insulation are covered with impervious layers; 
         [0029]    a glazing of the upper aperture of the housing (in some designs of the solar collector the glazing may be omitted); 
         [0030]    a solar radiation absorbing plate, which is fastened underneath the glazing in the housing and sealed with this housing or with the impervious side walls&#39; layers of the thermal insulation; 
         [0031]    a distributing pipe; the proximal section of this distributing pipe is placed outside the housing, and its middle and distal sections are installed on the backside of the solar radiation absorbing plate; the middle and distal sections of this distributing pipe are provided with openings (or nozzles), which supply evenly water or aqueous solution on the upper section of the backside of the solar radiation absorbing plate; the upper section of the absorbing plate backside is provided with some pipe clips serving for fastening the distributing pipe; 
         [0032]    rivulets&#39; flow restricting longitudinal means, which divide the backside of the solar radiation absorbing plate into a set of parallel zones; these rivulets&#39; flow restricting longitudinal means entrap the rivulets when they meet the rivulets&#39; flow restricting longitudinal means with following transformation of the shapes of these rivulets and flowing the rivulets in their transformed shapes along the rivulets&#39; flow restricting longitudinal means; 
         [0033]    an outlet connection, which is situated in lower section of the housing of the solar collector and serves for withdrawing the water, aqueous solutions or another liquid medium from the internal space of the flat solar collector (the space between the solar radiation absorbing plate and the impervious layer of the thermal insulation); 
         [0034]    a venting opening, which provides fluid communication of the internal space of the flat solar collector with the surrounding atmosphere. 
         [0035]    The flat solar collector, which is intended for evaporation and concentration of aqueous solutions, should be provided with an inlet connection and an outlet connection for supplying and removal of the air. In this case the venting opening may be lacking. 
         [0036]    The rivulets&#39; flow restricting longitudinal means can be designed on the base of several physical principles. 
         [0037]    These restricting means may operate on the base of capillary forces, gravitational force or by application of body force tangent to the substrate surface in opposite direction as a driving shear surface (see, for example, S Marshall and S. Wang CONTACT LINE FINGERING AND RIVULET FORMATION IN THE PRESENCE OF SURFACE CONTAMINATION, Computers &amp; Fluids, V. 34, Issue 6 Jul. 2006, pp. 664-683). 
         [0038]    It should be noted, that for very low values of flow rate a drop-wise flow can precede formation of rivulets&#39; flow. The invention proposes in this case the same flow restricting longitudinal means as for rivulets&#39; flow. 
         [0039]    In the first version of this invention there are a bank of strips which may be fabricated from any corrosion-resisting material (steel with anticorrosive coating, copper, thermo-stable polymer etc.). 
         [0040]    Each strip in this version is provided with one or two longitudinal beads and some openings with vertical flanging; an O-ring (or a back-up ring) is inserted into each opening; a tapered conical permanent magnet keeps close this O-ring (or back-up ring) against the opening flanging; the tapered conical permanent magnet is inserted into the opening until its immediate contact with the backside of the solar radiation absorbing plate with attendant deformation of the O-ring (or the back-up ring). 
         [0041]    In a simpler version of this technical solution, the strips are provided with some openings and O-rings (or back-up rings), which are placed concentrically with these openings; the diameter of each opening lies in the interval between the inner diameter of the O-ring and its outer diameter. 
         [0042]    A tapered conical permanent magnet, which has its top diameter somewhat smaller that the inner diameter of the O-ring and its base diameter somewhat larger that the inner diameter of the O-ring, is inserted with its tapered section into the opening until its immediate contact with the backside of the solar radiation absorbing plate. The top and base diameters of the O-ring are chosen in such a way, that this insertion causes radial extension of the O-ring, and friction forces between the tapered conical permanent magnet and this O-ring held in place the strip in contact with the backside of the solar radiation absorbing plate. 
         [0043]    In the next version of the invention the longitudinal strips, which are provided with two longitudinal beads (two lateral strips are provided with one bead each one), are designed as a grate with a frame comprising two lateral strips with one bead each one and upper and lower webs joining the beads of the neighboring longitudinal strips. 
         [0044]    The gap between the webs and the backside of the solar radiation absorbing plate is chosen in such a way, that it allows free entrance of the rivulets flowing on the backside of the solar radiation absorbing plate to the areas between the neighboring longitudinal strips. The grate may be fastened on the backside of the solar radiation absorbing plate by the same means as in the case of the longitudinal strips. 
         [0045]    The proposed solar collector should be installed with a certain angle of inclination to the horizontal plane. 
         [0046]    In addition, the upper and lower edges of the solar radiation absorbing plate of the installed solar collector may have a distinct angle of inclination with respect to the horizontal plane, this provides possibility that most of rivulets will contact with the rivulets&#39; flow restricting longitudinal means from a same side. 
         [0047]    The distributing pipe may be designed in such a manner, which ensures immediate contact of a supplied liquid medium (water, aqueous solution or thermo-stable organic liquid) with the upper section of the backside of the solar radiation absorbing plate. 
         [0048]    For example, the distributing pipe may be designed in a following form: the distal end of the distributing pipe is sealed and there is a set of openings in the wall of this distributing pipe, which are arranged in line. The wall openings are provided with outlet connections and the distal ends of the outlet connections are terminated with flexible sleeves; also, it allows installation of the distributing pipe on the backside of the absorbing plate in such a manner that the distal ends of the flexible sleeves will be in immediate contact with the backside of the solar radiation absorbing plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]      FIG. 1  demonstrates a vertical cross-section of a flat solar open-flow collector. 
           [0050]      FIG. 2 a   ,  FIG. 2 b    and  FIG. 2 c    are a top view and transverse cross-sections A-A and B-B of a grate-wise bank of strips with beads, wherein these strips are provided with openings. 
           [0051]      FIG. 3 a    and  FIG. 3 b    are cutaway transverse views of the solar radiation absorbing plate with fastening a strip ( FIG. 3 a   ) or a grate-wise bank of strips ( FIG. 3 b   ) by permanent magnets, wherein the strip or of the grate-wise bank of strips are provided with openings. 
           [0052]      FIG. 4 a    and  FIG. 4 b    are cutaway transverse views of the solar radiation absorbing plate with fastening a strip ( FIG. 4 a   ) or a grate-wise bank of strips ( FIG. 4 b   ) by permanent magnets, wherein the strip (or the grate-wise bank of strips) is provided with flanged openings. 
           [0053]      FIG. 5 a    and  FIG. 5 b    are cutaway transverse views of the solar radiation absorbing plate with fastening a strip with one bead ( FIG. 5 a   ) or a strip with two beads ( FIG. 5 b   ) by external permanent magnets. 
           [0054]    FIG. bis a cutaway transverse view of the upper section of the solar radiation absorbing plate with the installed distributing pipe. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0055]      FIG. 1  demonstrates a vertical cross-section of a flat solar open-flow collector  100 . 
         [0056]    It comprises: 
         [0057]    housing  101 ; 
         [0058]    thermal insulation layers  102 ; the internal surfaces of the layers of the thermal insulation  102  are covered with impervious layers; 
         [0059]    glazing  103  of the aperture of housing  101 ; 
         [0060]    a solar radiation absorbing plate  104  which is installed underneath glazing  103  and sealed with housing  101 ; the front side of the solar radiation absorbing plate  104  is provided with a solar radiation absorbing coating  105  and its backside—with a corrosion resisting coating  106 ; this solar radiation absorbing plate  104  is fabricated from ferromagnetic metal; 
         [0061]    a distributing pipe  108 ; the proximal section of this distributing pipe  108  is placed outside housing  101  and its middle and distal sections are situated underneath of the backside of the solar radiation absorbing plate  104 ; the distributing pipe  108  is installed on the backside by pipe clips  112 ; 
         [0062]    longitudinal strips  107  with beads  113 , openings  114  dividing the backside of the solar radiation absorbing plate  104  into a set of parallel zones; these longitudinal strips  107  with the beads  113  entrap the rivulets when they meet the longitudinal strips  107  with following flow of the rivulets along these longitudinal strips  107 ; permanent magnets  115  serves for fastening the longitudinal strips  107  on solar radiation absorbing plate  104 ; 
         [0063]    an outlet connection  110  which is situated at the bottom section of the internal space  111  of the solar collector  100  and serves for withdrawing the water or aqueous solutions; 
         [0064]    a venting opening  109  which provides fluid communication of the internal space  111  of the solar collector  100  with the surrounding atmosphere. 
         [0065]      FIG. 2 a   ,  FIG. 2 b    and  FIG. 2 c    are a top view and transverse cross-sections A-A and B-B of a grate-wise bank  200  of strips with beads, wherein these strips are provided with openings. 
         [0066]    It comprises: terminal strips  201  with beads  202 ; intervening strips  203  with their beads  204 ; webs  205 ; openings  206 , which are formed in terminal strips  201  and intervening strips  203 . 
         [0067]      FIG. 3 a    and  FIG. 3 b    are cutaway transverse views of the solar radiation absorbing plate with fastening a strip ( FIG. 3 a   ) or a grate-wise bank of strips ( FIG. 3 b   ) by permanent magnets, wherein the strip or of the grate-wise bank of strips are provided with openings. 
         [0068]      FIG. 3 a    comprises: a solar radiation absorbing plate  301  fabricated from ferromagnetic metal; the front side of this solar radiation absorbing plate  301  is provided with a solar radiation absorbing coating  302 ; the backside of the solar radiation absorbing plate  301  is provided with a corrosion resisting coating  303 . Strip  304 , which is fabricated corrosion resisting material, is installed on the backside of the solar radiation absorbing plate  301 ; this strip  304  is provided with two beads  305 . 
         [0069]    Strip  304  is provided with some openings  306 , which serve for installation of O-rings  307  and truncated conical permanent magnets  308 . 
         [0070]      FIG. 3 b    comprises: the solar radiation absorbing plate  301  fabricated from ferromagnetic metal; the front side of this solar radiation absorbing plate  301  is provided with the solar radiation absorbing coating  302 ; the backside of the solar radiation absorbing plate  301  is provided with the corrosion resisting coating  303 . Strip  310 , which is fabricated from corrosion resisting material and presents a section of a grate-wise bank of the strips, is installed on the backside of the solar radiation absorbing plate  301 ; this strip is provided with two beads  313 . Webs  311  serve for joining beads  313  of the aforementioned bank of strips  310 . 
         [0071]    Strip  310  is provided with some openings  312 , which serve for installation of O-rings  314  and truncated conical permanent magnets  315 . 
         [0072]      FIG. 4 a    and  FIG. 4 b    are cutaway transverse views of a solar radiation absorbing plate with fastening a strip ( FIG. 4 a   ) or a grate-wise bank of strips ( FIG. 4 b   ) by permanent magnets, wherein the strip or the grate-wise bank of the strips are provided with flanged openings. 
         [0073]      FIG. 4 a    comprises: the solar radiation absorbing plate  401  fabricated from ferromagnetic metal; the front side of this solar radiation absorbing plate  401  is provided with a radiation absorbing coating  402 ; the backside of the solar radiation absorbing plate  401  is provided with a corrosion resisting coating  403 . Strip  404 , which is fabricated from corrosion resisting material, is installed on the backside of the absorbing plate  401 ; this strip is provided with two beads  405 . Strip  404  is provided with some openings with outward flanges  406 , which serve for installation of O-rings  408  and truncated conical permanent magnets  409 . 
         [0074]      FIG. 4 b    comprises: the solar radiation absorbing plate  401  fabricated from ferromagnetic metal; the front side of this solar radiation absorbing plate  401  is provided with the radiation absorbing coating  402 ; the backside of the solar radiation absorbing plate  401  is provided with the corrosion resisting coating  403 . Strip  410 , which is fabricated from corrosion resisting material and presents a section of a grate-wise bank of such strips, is installed on the backside of the absorbing plate  401 ; this strip is provided with two beads  411 . Webs  412  serve for joining beads  411  of the aforementioned bank of strips  410 . 
         [0075]    Strip  410  is provided with some openings with outward flanges  413 , which serve for installation of O-rings  415  and truncated conical permanent magnet  416 . 
         [0076]      FIG. 5 a    and  FIG. 5 b    are cutaway transverse views of a solar radiation absorbing plate  501  with fastening a strip with one bead ( FIG. 5 a   ) or a strip with two beads ( FIG. 5 b   ) by external permanent magnets. 
         [0077]      FIG. 5 a    comprises: the solar radiation absorbing plate  501  fabricated from ferromagnetic metal; the front side of this solar radiation absorbing plate  501  is provided with a solar radiation absorbing coating  502 ; the backside of the solar radiation absorbing plate  501  is provided with a corrosion resisting coating  503 . Strip  504 , which is fabricated from ferromagnetic steel, is installed on the backside of the solar radiation absorbing plate  501 ; this strip is provided with one bead  505  and covered (including bead  505 ) with a corrosion resisting coating  506 . Strip  504  is secured on the solar radiation absorbing plate  501  by a permanent magnet  507 . 
         [0078]      FIG. 5 b    comprises: the solar radiation absorbing plate  501  fabricated from ferromagnetic metal; the front side of this solar radiation absorbing plate  501  is provided with the radiation absorbing coating  502 ; the backside of the solar radiation absorbing plate  501  is provided with the corrosion resisting coating  503 . Strip  508 , which is fabricated from ferromagnetic metal, is installed on the backside of the solar radiation absorbing plate  501 ; this strip is provided with two beads  509  and covered (including beads  509 ) with a corrosion resisting coating  510 . Strip  508  is secured on the solar radiation absorbing plate  501  by a permanent magnet  511 . 
         [0079]    FIG. bis a cutaway transverse view of the upper section of a solar radiation absorbing plate  601  with a distributing pipe  604 , which is installed on the upper section of the backside of the solar radiation absorbing plate  601 . 
         [0080]    The drawing depicts: the solar radiation absorbing plate  601 ; the front side of this absorbing plate  601  is provided with a radiation absorbing coating  602 ; the backside of the solar radiation absorbing plate  601  is provided with a corrosion resisting coating  603 ; the distributing pipe  604  is provided with nozzles  605 , which are terminated with flexible sleeves  606 ; pipe clips  607  are installed on the backside of the absorbing plate and serve for securing the distributing pipe  604 .