Abstract:
A direct flow solar collector and solar hot water system are presented wherein high pressure connections are eliminated to lower installation costs while freeze and stagnation protection is provided by a cooling loop and a continuous circulation protocol. A novel fin design and a modular concept deliver manufacturing, shipping and assembly efficiencies while providing flexibility for customizing the collector configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuing application of co-pending International Application PCT/US2013/065807, filed 20 Oct. 2013, and a continuation-in-part application of co-pending International Application PCT/US2013/027339, filed 22 Feb. 2013, with combined priority claims to U.S. Provisional Application 61/603,541, filed 27 Feb. 2012, U.S. Provisional Application 61/660,446, filed 15 Jun. 2012 and U.S. Provisional Application 61/716,727, filed 22 Oct. 2012, the aforementioned PCT/US2013/065807 additionally with a priority claim to the aforementioned PCT/US2013/027339, all the foregoing of which are incorporated herein by reference in entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the harnessing of solar energy, and more particularly to solar hot water heating. 
       BACKGROUND OF THE INVENTION 
       [0003]    Concerns with the aging infrastructure of the electricity grid in the United States, the environmental impact of carbon-based fuels, the geo-political ramifications of crude oil supply, the depletion of natural resources, the safety concerns related to nuclear power, and the distribution and extraction challenges surrounding natural gas have all brought increasing attention to safe alternative energy sources such as Solar. Solar energy can be captured as heat as well as converted to electricity. Next to space heating, water heating is the most significant use of this virtually inexhaustible and universally available resource. 
         [0004]    Radiation from the sun is captured by a hot water solar collector, which is generally a part of a system comprising a water supply and/or storage means, a circulation pump, and a heat-exchanger means. Flat-plate solar collectors are popular in the sunny, year-around warm, climates in the United States. They are comprised of a heat-absorption panel in contact with circulating water and are typically housed under a transparent covering and over an insulating bed. The flat panels absorb solar energy in much the way photo-voltaic (PV) electric panels do. Heat transfer efficiency is a function of reflective, conductive, and convective heat losses, as well as the temperature differential between the absorption panel and the incident water. While highly efficient where optimally located, flat-plate collectors give poor performance, nevertheless, in high temperature applications and in cold conditions where high heat losses, due to the large surface exposure of the panels, are experienced. Furthermore, maintenance is cumbersome and often involves shutting the system down or replacing the entire collector. 
         [0005]    Evacuated tube (EVT) collectors are common in China, where 60% of global solar collector capacity is installed and where over 70% of the EVT&#39;s are manufactured. The most popular EVT type is a double-walled glass tube having an evacuated air space between the walls. The inside tube is coated to enhance absorption and back-reflection of spectral infra-red (IR). Heat absorbed by the tubes is transferred to a manifold, into which they are inserted, by circulation of a liquid through an internal viaduct (U-tube) or by phase change of a captive liquid in a capillary (heat pipe). The tubular design accommodates different solar angles. The use of a complete vacuum as an insulator makes them appropriate for cold climates and where extremely high temperatures are required or maintained. Maintenance is also simpler, in that individual tubes can be replaced, often while the system is in use. 
         [0006]    The major disadvantage of the EVT collector is the cost in dollars per BTU. Considering the disadvantages of both the flat-plate and EVT types of collectors, the northern climates of the United States, where freezing temperatures are an issue for much of the year, and low solar angles and short days are prevalent for a portion of the year, are left under-served by solar hot water. The same holds true for other colder locations throughout the world where the economics of cheaper energy sources prevail. 
         [0007]    The object of the present invention is to enable solar hot water heating in both cold and warm climates by making the installation of a solar hot water system less expensive and the operation thereof more efficient. These objects are achieved by comprehensively addressing cost elements in the construction, configuration and operation of the EVT collector. 
       SUMMARY OF THE INVENTION 
       [0008]    The heat pipe EVT design requires a heat exchange in the manifold, whereas the U-tube design avoids this exchange and the energy losses attendant thereto by directly heating the solar liquid. The disadvantages of the currently-practiced U-tube technology, however, present opportunities for improvement. Both freezing and overheating, also called “stagnation”, become issues with the solar liquid present in the EVT. Typically, to protect from freezing, glycol antifreeze is used for the solar liquid in a closed-loop circulation system. High pressure is required to prevent the glycol from boiling during periods of stagnation. To heat water in a storage tank, or a swimming pool, the glycol must communicate through a heat exchanger, presenting another opportunity for thermal loss. 
         [0009]    The high pressure requirement drives high installation costs. The U-tubes are typically arranged in an array and integrated into a manifold configuration at a factory location. Typically, 30-40 U-tubes are joined by brazing, soldering or welding and then shipped to the installation site in an out-sized shipping configuration. Shipping costs associated with such a configuration can be expensive. 
         [0010]    The substitution of water for glycol as the solar liquid eliminates the high pressure requirement. Elimination of the high pressure requirement means less expense in materials, shipping, and assembly. In an alternative low pressure system, seals and flexible tubing can be used to join components. Eliminating the factory configuration means that modular design can be implemented and customization facilitated. Assembly work can be done on-location with unskilled labor. Pressure and relief tanks can be eliminated as necessary system components, and the higher cost of pressure-rated pumps can be avoided. System maintenance can be achieved without bleeding and recharging lines and without incurring the loss of a relatively expensive solar liquid, such as glycol. 
         [0011]    Freezing and stagnation remain as issues, however, with water-based solar liquid. This is partially addressed in the present art by gravity drain-back systems, which prevent freezing by evacuating the solar liquid channels. Typically, however, some liquid is always left fugitive in the channels, regardless of the orientation or tilt of the collector array. These “gravity blind-spots”, or pathway low spots, can cause flash vaporization during stagnation periods, thereby causing damage to the EVT&#39;s. Even when drain-back systems are purged by air flushes, or other means, the high temperatures of stagnation can cause damage to system components. This damage can be particularly devastating to plastic, rubber and aluminum parts used in low-pressure systems. 
         [0012]    The present invention presents the novel concept of “dumping heat” by continuous circulation of the solar liquid during non-heating periods and by the use of supplemental cooling during over-heating periods. With a sufficiently large storage tank as a heat repository, it has been shown that water in the tank can be maintained in a range of 40-65° C., even in a high solar fraction climate, such as Mexico. Since the heat gained is essentially free, the wasting of it is not an economic matter, regardless of efficiency loss, particularly if the waste heat is simply vented to the environment. In colder climates, the maintenance of constant circulation between the solar collector and an appropriately-sized storage tank can prevent freezing, particularly where exposed plumbing is insulated. 
         [0013]    An additional novel concept facilitates lowering pressure and enhancing the thermal efficiency of the U-tube. The concept addresses the fin, which is commonly attached to standard tubing and projects arcuate “wings” there from which serve to absorb radiant heat passing through the EVT walls. In the concept, the fin and the water channel are combined in a single extrusion, which is then bent into a “U” shape. The space between the upright arms of the “U” is filled with resilient insulating material, which has the effect of spreading the arms outwardly and urging them against the EVT wall. The construction eliminates the insulating air gap that results from separate components, and the curvature of the extrusion, by closely following the wall contour, adds additional thermal efficiency by permitting large area surface contact with the wall. 
         [0014]    Further, the cross-sectional area of the water channel is enlarged by changing by changing from a circular to an ellipsoidal shape. The enlargement, measuring almost twice that of the standard tube, permits a larger volume of throughput resulting in lower pressure. With system pressure reduced, not only by eliminating glycol but also by making the channel enlargement, inexpensive and easy-to-install flexible tubing may be used in the interconnecting links of the U-tubes. 
         [0015]    Still another novel concept derives from manipulation of series and parallel connections channeling the water through the U-tubes. U-tubes connected in series provide higher output temperatures, but too many tubes connected in this manner produce lower heat-transfer efficiency because successive heating causes reduction of the temperature differential, the thermal driving force. On the other hand, U-tubes connected in parallel have a constant temperature differential but require a longer circulation and pump operation time to achieve the same heating result. An unexpected result of the parallel configuration, however, is that the necessarily larger supply channels tend to cause a cavitation effect in the relatively smaller tube channels; that is, air pockets get trapped that produce an insulating layer and result ultimately in lowered thermal efficiency. The inventive concept is to use an optimal combination of both configurations to give an improved result in terms of operating cost per BTU. 
         [0016]    It is accordingly an object of the present invention to lower the system pressure requirements by using a U-tube design in an atmospheric system with water as the solar liquid. It is a further object to eliminate drain-back systems by continuously circulating the solar liquid and providing supplemental cooling. It is a further object to prevent freezing of the solar liquid by trickle circulation and the selective insulation of water channel lines. It is a further object to lower assembly and material costs by using flexible tubing connections. It is a further object to improve thermal efficiency by integrating the water channel and the fin in the same structure to eliminate any air gap in otherwise separate structures. It is a further object to improve thermal efficiency by making surface-to-surface contact between the fin and EVT inner wall. It is a further object to enlarge the U-tube water channel to lower the tube-to-tube pressure drop. It is a further object to use a combination of series and parallel connections in the water channels to optimize thermal efficiency. It is a further object to make more efficient use of roof-top space by reducing tube-to-tube spacing in the manifold. It is a further object to manage roof-top space by configuring the EVT array with tubes on either or both sides of the manifold as dictated by site layout. It is a further object to manage roof-top space and improve appearance by permitting low angle or horizontal installation profiles essentially hidden from view. It is a further object to facilitate snow removal from the collector. It is a further object to provide a solar hot water system incorporating a hot water storage vessel. It is a further object to provide a method of configuring a solar collector to achieve operating efficiency. 
         [0017]    These objects, and others to become hereinafter apparent, are embodied in a solar collector comprising a manifold having an input port, an output port and a plurality of orifices. The solar collector further comprises a corresponding plurality of solar tubes connected to the manifold through the orifices, the plurality of solar tubes assembled in a planar array and positioned for exposure to solar radiation. The solar collector further comprises at least one liquid channel from the input port to the output port. The liquid channel has at least one continuous flow path there through. The solar collector further comprises a means for transferring heat absorbed from solar radiation in each solar tube to a solar liquid flowing through the at least one continuous flow path. The solar collector further comprises a means for circulating the solar liquid through the means for transferring heat. Finally, the solar collector comprises a means for cooling the solar liquid to maintain its temperature at or below a preferred temperature. So configured, heat from solar radiation is transported for work purposes through the solar liquid by the means for transferring heat and by the means for circulating and the solar liquid is prevented from overheating by the means for cooling. 
         [0018]    In one embodiment, the means for transferring heat comprises a fin inserted into each solar tube and extending the length of the tube. The fin has an integrated U-shaped channel extending from an input end to an output end. The input end of a first U-shaped channel in a first solar tube of the planar array forms a fluid connection to the input port while the output end of the first U-shaped channel of the first solar tube forms a fluid connection in preferred configuration to the input end of a last U-shaped channel in a last solar tube. The output end of the last U-shaped channel in the last solar tube then forms a fluid connection to the output port. So configured, at least one liquid channel is formed for a continuous flow path through each solar tube, the flow of solar liquid therein receiving heat by conduction from the fin. In a particular instance of the embodiment, the preferred configuration is a serial linkage joining ten solar tubes and a parallel linkage joining four serial linkages. 
         [0019]    Also in the embodiment, the means for cooling comprises a cooling loop and a controller. The controller is programmed to redirect circulation of the solar liquid through the cooling loop when the solar liquid is above a first preferred temperature. In a particular instance of the embodiment, the first preferred temperature is in the range of 55-60° C. 
         [0020]    In the preferred embodiment, a solar hot water system comprises a solar collector having a manifold, a plurality of solar tubes, at least one liquid channel, and a means for transferring heat, as discussed above. The solar hot water system further comprises a storage vessel for hot water in fluid communication with the input port and the output port of the manifold. The solar hot water system further comprises a means for circulating the solar liquid through the means for transferring heat to the storage vessel. Lastly, the solar hot water system comprises a means for cooling the solar liquid to maintain its temperature at or below a preferred temperature. So configured, the heat from solar radiation if used to heat the water in the storage vessel by the means for transferring heat and by the means for circulating and the solar liquid is prevented from overheating by the means for cooling. In a particular instance of the preferred embodiment, the solar hot water system further comprises a means for preventing freezing of the solar liquid. 
         [0021]    As this is not intended to be an exhaustive recitation, other embodiments are described in the detailed description below; or may be learned from practicing the invention; or may otherwise become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0022]    Various other objects, features and attendant advantages of the present invention will become fully appreciated as the same becomes better understood through the accompanying drawings and the following detailed description, in which like reference characters designate the same or similar parts throughout the several views, and wherein: 
           [0023]      FIG. 1  is a perspective view of a mounted solar collector of the present invention shown in a bi-lateral configuration; 
           [0024]      FIG. 2  is a truncated elevation view of an EVT; 
           [0025]      FIG. 3  is a section view of the EVT taken along the lines  3 - 3  of  FIG. 2 ; 
           [0026]      FIG. 4  is a partial perspective view of two serially connected U-tubes; 
           [0027]      FIG. 5  is a detail view of detail  5  in  FIG. 4 ; 
           [0028]      FIG. 6  is a detail view of detail  6  in  FIG. 4 ; 
           [0029]      FIG. 7  is a partial perspective view of a 20-tube array showing series and parallel path connections; 
           [0030]      FIG. 8  is a partial plan view of a 20-tube array showing a modular connection to tubes  21  and  22  in phantom line; 
           [0031]      FIG. 9  is a side view of a 10-tube module with an eleventh tube in phantom line; 
           [0032]      FIG. 10  is a truncated section view taken along lines  10 - 10  of  FIG. 9  showing solar liquid paths; 
           [0033]      FIG. 11  is a partial exploded perspective view of three serially-linked tubes; 
           [0034]      FIG. 12  is a truncated perspective view of an extruded fin; 
           [0035]      FIG. 13  is a truncated elevation view of a U-tube 
           [0036]      FIG. 14  is a section view taken along the lines  14 - 14  of  FIG. 13  showing a cross-section of the water channel; 
           [0037]      FIG. 15  is a perspective view of the manifold; 
           [0038]      FIG. 16  is an exploded perspective view of the manifold and insulation; 
           [0039]      FIG. 17  is a diagram of a solar collector system of the present invention; and 
           [0040]      FIG. 18  is a perspective view of the mounted solar collector of  FIG. 1  shown in a unilateral configuration. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]    Flow paths referenced in the specification are illustrated throughout the drawings by bolded arrows (other than those indicating sectional cuts). 
         [0042]      FIGS. 1 and 18  show the major components of a solar collector  1 . A solar tube planar array  12  is connected to a manifold  20  through orifices  29 . The manifold  20  has an input port  22  and an output port  23  defining there through and there between at least one liquid channel  4  receiving circulation of a solar liquid  3  (see  FIG. 17 ) through at least one continuous flow path  9  (see also  FIG. 10 ). When the planar array  12  is mounted to receive solar radiation, such as on the roof top of a building, the solar collector  1  may be supported by frame members  2  received in frame slots  100  located in the middle and both ends of the solar collector  1  (see also  FIGS. 6 and 15 ). 
         [0043]    As shown in  FIGS. 7 and 8 , the solar tube planar array  12  is comprised of a plurality of solar tubes  7 . The planar array  12  may be arrayed from opposing sides of the manifold  20 , or bi-laterally, as shown in  FIG. 1 . Alternatively, the planar array  12  may be configured only on one side, or unilaterally, as shown in  FIG. 18 . The latter configuration facilitates snow removal, in addition to accommodating site-particular space constraints. When arrayed bi-laterally, the tubes are typically oriented east-west; when arrayed unilaterally, the orientation is north-south. The array should be inclined to present roughly perpendicular to the sun&#39;s rays at the installation latitude. In most cases, the inclination angle  106  is in the range 0-18° plus the solar angle ( FIG. 1 ). At a high solar angle, the profile elevation can be as little as 25 cm. The frame members  2  may be directly anchored to a supporting surface by any fastening means, or may be moveably held in-place by a weighted base (not shown). An approximate weight for a sufficiently-weighted base is 25 kg. 
         [0044]    The number of solar tubes  7  in the planar array may be limited by roof-top layout or, otherwise, by the design pressure drop across the circulatory pathway. In the preferred embodiment, it is desirable to maintain a low operating pressure for cost advantage reasons. In a particular preferred embodiment, it is the objective to maintain the operating pressure at 0.14-0.70 kgf/cm 2  (2-10 psi) and a flow rate of 5.68 1 pm (1.5 gpm). Accordingly, an optimal array would be comprised of not more than 40 tubes. Such an array has a footprint approximately 47% smaller than a thermally-equivalent flat plate collector. 
         [0045]    The solar tube  7  is designed to receive solar radiation through a glass envelope and retain the energy as heat in the interior. In the preferred embodiment, the solar tube  7  is a double-walled EVT  8 , as shown in  FIGS. 2-6 . There is a vacuum space  101  in EVT  8  between an inner tube  102  and an outer tube  103 , the vacuum serving as an insulator for entrapped heat. The inner tube  102  has several coating layers to enhance performance, namely an anti-reflection layer, an absorbance-enhancing layer and an IR reflection layer. The EVT  8  has an open end  104  and a closed end  109 . When the solar tube  7  is connected to the manifold  20  through one of the orifices  29 , the open end  104  shoulders against a ledge  105  in the interior ( FIG. 15 ). The closed end  109  is cradled in an end cup  17 , which is supported in the mounted configuration of solar collector  1  by an end cup support  18 . The end cup support  18  has an adjustment screw  107 , which serves to bias the open end  104  against the ledge  105 . The solar tubes  7  may be individually removed and replaced in the array by disconnecting the end cup  17  from the end cup support  18 . 
         [0046]    The at least one liquid channel  4  is comprised of a means for transferring heat  30 , as shown in  FIGS. 11-14 . In the preferred embodiment, said means comprises a fin  31 . Fin  31  has an integrated U-shaped channel  32 , which forms a part of the at least one continuous flow path  9 . The integration of the U-shaped channel  32  into the fin  31  eliminates thermal losses resulting from air gaps between otherwise separate structures. Fin  31  is inserted into the EVT  8  to the extent of the draw therein. Fin  31  has arcuate wing members  108  flanking the U-shaped channel  32  to form a contact surface with the inner tube  102  through which heat is conducted to the solar liquid  3  flowing through the U-shaped channel  32 . The arms of the “U” of the U-shaped channel  32  may be biased outwardly by a resilient insulation plug  44  (see also  FIG. 5 ) to make contact with the wall of the tube and thereby eliminate another potential air gap. Fin  31  is fabricated as an extrusion  36  and is comprised of aluminum. Arcuate wing members  108  may be trimmed away ( FIG. 12 ) to form nipples  39  of U-shaped channel  32  at either end thereof. Two such nipples  39  may be joined in a U-configuration by a U-shaped connector  109  (not shown). The U-shaped connector  109  may be, for example, a compression-molded piece of silicone. In the preferred embodiment, the U-configuration is achieved by bending a single extrusion with slotted wings into the U-shaped channel  32 . The U-shaped channel  32  maintains its throat by means of gussets  35  therein. 
         [0047]    When fin  31  is inserted into EVT  8  ( FIG. 11 ), two nipples  39  protrude from the open end  104  of the solar tube  7  to form an input end  33  and an output end  34  of U-shaped channel  32 , as shown in  FIG. 10 . The output end  34  in one solar tube  7  may be connected to the input end  33  in another solar tube to form a serial linkage  41 . The input end  33  in one solar tube may also be connected to the input end in another solar tube to form a parallel linkage  42 . The input end  33  of a first U-shaped channel  37  in a first solar tube  5  of the planar array  12  is connected to the input port  22  of the manifold  20  and the output end  34  of a last U-shaped channel  38  in a last solar tube  6  is connected to the output port  23 . The input port  22  is connected to the output  23  in a preferred configuration  40  comprised of a preferred number of solar tubes  7  in the serial linkage  41  and a preferred number of serial linkages  41  in a parallel linkages  42  to form the at least one liquid channel  4  through which the solar liquid  3  may flow through each solar tube  7  in the at least one continuous flow path  9 . Because the at least one liquid channel  4  is contained within the fins  31 , the solar tubes  7  may be changed out for repair or replacement, in the manner discussed above, without affecting the operation of the solar collector. 
         [0048]    One objective in the optimization of thermal efficiency is to avoid non-turbulent flow in the at least one continuous flow path  9 . This is achieved by using serial linkage  41 , wherein the cross-sectional area of the at least one liquid channel  4  can more closely approximate that of the cross-sectional area of the U-shaped channel  32 . Another objective in the optimization of thermal efficiency is to maintain at least some temperature differential across the path through serial linkage  41 . This requires limiting the number of solar tubes  7  in any one serial linkage  41  and connecting multiple serial linkages  41  in the parallel linkage  42 . The span of the parallel linkage  42 , in terms of the number of tubes, is determined by the optimal pressure drop across the at least one continuous flow path  9 , which in turn defines the operating pressure of the system. In the preferred embodiment, the cross-sectional area of the U-shaped channel  32  is approximately 72 mm 2 . This area compares favorably with the approximately 36 mm 2  opening of the standard tube of current art. The preferred embodiment comprises ten solar tubes  7  to a serial linkage  41  and four serial linkages  41  to a parallel linkage  42 , optimally defining the planar array  12  as an array of 40 tubes (the first  10  tubes only are shown in  FIG. 10 ). It has been discovered that optimal thermal efficiency, as measured by cost per BTU, occurs by balancing the non-turbulent flow consideration, having the consequence of avoiding insulating air pockets in the channel, with the pressure consideration, having the consequence of protecting temperature differential. The thermal performance of the 40-tube array herein described is roughly equivalent to two 1.22 m×3.05 m (4′×10′) flat plate collectors. The pressure consideration also affects other efficiencies of construction. 
         [0049]    One of these other efficiencies afforded by low operating pressure is the use of flexible tubing  10 . Flexible tubing  10  is inexpensive and simplifies installation.  FIGS. 7 and 8  show a network of the flexible tubing  10  in both serial and parallel configurations. In the preferred embodiment, the flexible tubing  10  is comprised of high temperature (rated at 250° C.) silicone rubber. The flexible tubing  10  forms a seal with the nipples  39  when compressed thereon by a band, clip, or other form of compression known in the art. As shown in solid line in  FIG. 8 , a modular unit of planar array  12  is comprised of 20 solar tubes  7 . As shown in dashed line, two or more modular units may be combined by extending parallel linkage  42  through linkage sections  13  of flexible tubing  10 . The modularity of the design facilitates customized installation and delivers cost benefits associated with on-site assembly. 
         [0050]    The manifold  20  is comprised of manifold housing assembly  21  and insulation core  25 , as shown in  FIGS. 15 and 16 . The manifold housing assembly  21  is comprised of a housing base  26  and a housing top  27 . The housing top  27  is connected to the housing base  26  by a manifold seal  24  at each side to form an enclosure. Each manifold seal  24  comprises a plurality of orifices  29  to receive the solar tubes  7  of the planar array  12 . The insulation core  25  has a center bore  28  in which the at least one liquid channel  4  is situated, wherein the function of said insulation core is to insulate the liquid channel  4  from heat loss. The housing base  26  and the housing top  27  are fabricated from aluminum by extrusion means. The manifold seal  24  is fabricated in a molding of ethylene propylene diene monomer (EPDM) material. The insulation core  25  is fabricated from sponge rubber material. 
         [0051]    The solar collector  1  further comprises a means for circulating  60  the solar liquid  3  through the means for transferring heat  30 . In the preferred embodiment, the means for circulating  60  comprises a first low-pressure pump  61 , a solar liquid loop  74  and a holding tank  65 , as shown in  FIG. 17 . The holding tank  65  serves as a reservoir for the solar liquid  3 , which may be supplied to the reservoir from another source or may circulate in a closed loop therein. The solar liquid loop  74  communicates with the holding tank  65  and includes the supply path  59  and the return path  58 . The first low-pressure pump  61  is controlled by controller  55 . 
         [0052]    The solar collector  1  further comprises a means for cooling  50  the solar liquid  3  to prevent over-heating. The means for cooling  50  is initiated when the solar liquid  3  reaches a preferred temperature  51 . The means for cooling  50  may include refrigeration of, or immersion of, the solar liquid loop  74  in an air stream or a body of water, such as a pool or lake. The means for cooling  50  may also include cycling the first low-pressure pump  61  during nighttime or overcast days. In the preferred embodiment, the means for cooling  50  comprises the controller  55  and a cooling loop  52 , as shown in  FIG. 17 . When the solar liquid  3  reaches a first preferred temperature  53 , the controller  55  initiates circulation of the solar liquid in the holding tank  65  through the cooling loop  52  by activating a second low-pressure pump  62 . In an instance of the preferred embodiment, the first preferred temperature  53  is in the range of 55-60° C. The cooling loop  52  essentially vents heat by convection to a relatively cooler environment, such as may be found in an underground vault. In the preferred embodiment, the cooling loop  52  is comprised of a serpentine configuration of stainless steel tubes linked with silicone tubing. The controller  55  is in signal communication with one or more sensors  64  positioned at selected locations throughout the solar liquid loop  74 . The sensors  64  may be transducers or thermocouples and may measure temperature or pressure or both. In the preferred embodiment, at least one of the sensors  64  is located in the holding tank  65  and measures the temperature of the solar liquid  3 . 
         [0053]    The solar collector  1  further comprises a means for preventing freezing  80  of the solar liquid  3  in the exposed portion of the solar collector. The means for preventing freezing  80  comprises the continuous circulation of the solar liquid during low temperatures. In the preferred embodiment, the controller  55  activates the first low-pressure pump  61  when the solar liquid  3  drops below a second preferred temperature  81 . In an instance of the preferred embodiment, the second preferred temperature  81  is in the range of 2-5° C. The means for preventing freezing  80  further comprises insulation of the exposed portions of the solar liquid loop  74 . In the preferred embodiment, an insulation wrap  82 , packed within a flexible polypropylene (PP) hose, surrounds the flexible tubing  10  leading to and from the input and output ports ( FIG. 1 ). The insulation wrap  82  may be comprised of sponge rubber, or any other known insulating material. The means for preventing freezing  80  additionally includes the resilient insulation plugs  44  positioned inside the solar tubes  7 . 
         [0054]    In the preferred embodiment, the holding tank  65  is a hot water storage vessel  71 , as shown in  FIG. 17 . Hot water storage vessel  71  is a member of a solar hot water system  70 , which is also includes solar collector  1 . In an instance of the preferred embodiment, hot water storage vessel  71  is a roto-cast tank comprised of crossed-lined high-density polyethylene (HDPE). In another instance, the solar liquid  3  of the solar hot water system  70  is water  73  stored in water storage vessel  71 . Preferably, the water  73  is of neutral pH and may contain stabilizing or anti-corrosion additives. The means for circulating  60  further comprises a circulation of water  73  from a hot zone  76  of the water storage vessel  71  to a cold zone  77 . The water storage vessel  71  is preferably large enough in volume for a stratification to occur by the colder, denser water gravitating downward. The water storage vessel  71  is also preferably large enough to retain heat during extended non-solar periods. In the preferred embodiment, the water storage vessel  71  is jacketed with insulation. The insulation may be comprised of polyurethane foam, or other known insulating material. The solar liquid loop  74  fluidly connects the cold zone  77  to the input port  22  and the hot zone  76  to the output port  23 . The cooling loop  52  circulates in and out of the hot zone  62 . 
         [0055]    Hot water storage vessel  71  may further comprise submerged heat exchanger  72 . The heat exchanger  72  effectively removes heat from the storage part of the system. In the case of swimming pool heating, chlorinated pool water may be heated in the heat exchanger  72  without contaminating the solar liquid  3 . For domestic hot water use, the heat exchanger  72  outputs hot water on demand from a pressurized cold water intake-line. In the preferred embodiment, heat exchanger  72  is comprises of stainless steel tubing configured into a spiral and is submerged in the solar liquid  3 . 
         [0056]    In an alternative embodiment, a method of configuring a solar collector to achieve operating efficiency, as measured by cost per BTU, comprises the steps as follows:
       a) providing the solar collector  1 , wherein the means for transferring heat  30  is a fin  31  inserted into each solar tube  7  and extending the length thereof, said fin having an integrated U-shaped channel  32  extending from an input end  33  to an output end  34 ; the input end  33  of a first integrated U-shaped channel  37  in a first solar tube  5  of the planar array  12  forming a fluid connection to the input port  22 ; the output end  23  of the first U-shaped channel  37  in the first solar tube  5  forming a fluid connection in a preferred configuration  40  to the input end  33  of a last U-shaped channel  38  in a last solar tube  6 ; and the output end  34  of the last U-shaped channel  38  in the last solar tube  6  forming a fluid connection to the output port  23 ;   b) implementing the preferred configuration  40  by joining a preferred number of solar tubes  7  in a serial linkage  41 , said serial linkage  41  balancing heat transfer efficiency with non-turbulent hydraulic flow;   c) implementing the preferred configuration  40  by joining a preferred number of serial linkages  41  in a parallel linkage  42 , said parallel linkage  42  balancing heat transfer efficiency with hydraulic pressure.       
 
         [0060]    It is to be understood that the invention is not limited in its application to the details of construction, to the arrangements of the components and to the method of using set forth in the preceding description or illustrated in the drawings. For example, the serial linkage count may be greater than ten to provide higher temperatures; or the parallel linkage count may be greater than 4 to provide an increased solar fraction in colder climates. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of the description and should not be regarded as limiting.