As is well known, solar energy collectors operate by absorbing heat from the sun into water or other suitable fluid flowing through the collector.  Primarily, the fluid is heated by circulating it through an array of thermally conductive pipes which are directly exposed to the sun's radiant energy.  The fluid to be heated first flows into the array from a standard source, is heated while passing through the array, and is thereafter collected for subsequent harvesting of the contained heat in accordance with standard solar energy methodology.  A common use of such solar collectors is to heat water supplies for residential, commercial and/or industrial hot water consumption. 
The various major elements of solar water heaters comprise the absorber, i.e. that portion of the heater which directly receives the sun's rays and transfers the received heat energy to water or other fluids running through connected pipes or risers; the collector, i.e., the structure encapsulating the absorber in an insulated box-like frame with a transparent top; and the water storage tank usually distant from the collector. 
There are presently two basic types of solar water heaters; namely, the open loop and the closed loop systems.  In the closed loop system, water or antifreeze or other suitable fluid is continuously circulated between a solar energy collector usually disposed on a building roof and a heat exchanger which is disposed in operative association with a water storage tank, usually inside the building.  The water to be heated for residential or commercial use is brought into thermal contact with the heat exchanger and receives heat energy from the continuously circulating medium which has been previously passed through the solar collector on the building roof. 
In the open loop system, the actual water to be used is circulated through the solar collector and thereafter may then be blended with unheated water to attain a desired temperature balance and thereafter distributed as hot water to its point of use.  Either system may temporarily store heated water in a storage tank which is disposed in line with the building water supply.  The present invention is useful with either an open or a closed loop system. 
Construction of the absorber as generally practiced involves forming a series of parallel semi-circular grooves in a piece of flat sheet metal, such as sheet copper, and then placing individual cross-runs or risers of copper pipe into these semi-circular grooves.  The pipes were physically soldered to the sheet metal.  Generally the sheet metal was rectangular in shape, being 3 to 4 feet in height and 8 to 10 feet in panel or sheet of the metal.  The panel, when installed, was aligned longways, i.e., one of the length sides resting upon the roof oriented to face the sun, sloped at an angel to the horizon, and painted black to more efficiently absorb the sun's rays.  The copper pipes were usually on the underside (away from the sun) of the absorber panel. 
More specifically however, the present invention is directed toward the improvement of those solar collectors which comprise a plurality of parallel, elongated riser tubes connected to each other at their respective ends with elbow or tee joints using standard soldering or welding techniques and arranged in heat conductive relationship with radiation-receiving collector panels.  Elbow joints connect the riser tubes in a serpentine fashion and the water travels serially through the several riser tubes, reversing directions as it flows through adjacent tubes.  Tee joints, on the other hand, are used for manifold-type collectors in which the water is introduced into an inlet manifold operatively connected to all of the riser tubes.  Water or other heat transfer medium is caused to flow into a manifold-type collector and then passed in parallel paths simultaneously through all of the riser tubes to a discharge manifold positioned on the opposite end of the collector.  The heated water is then drawn from the second manifold for storage and/or use. 
In both systems, thermally conductive, pliable, and usually metallic sheets or panels which receive and absorb the energy from the sun in the form of heat are mounted between and connected to all of the elongated riser tubes.  This solar heat is then conductively transferred from the collector panels to the riser tubes and hence to the fluid medium flowing therethrough.  In use, these systems and their interconnected assembly of collector plates and riser tubes gave rise to a number of leaks and cracks which occurred from the repeated longitudinal expansion/contraction of the riser tubes relative to the associated elbows/manifolds. 
As described in Reed, U.S. Pat.  No.  5,074,282, these types of solar collectors were originally formed from a single, large panel of sheet metal, approximately 8 to 10 feet long by 3 to 4 feet wide, which was arrayed with multiple riser tubes running along the length of the panel and disposed in spaced generally parallel relationship to each other.  Copper was the material of choice for both the riser tubes and the panel.  Semicircular grooves were formed in the panel to receive the riser tubes in an effort to create good thermal contact therebetween on the theory that the greater the area of actual surface-to-surface contact, the more efficient the heat exchange.  To effect a secure thermal contact between the riser tubes and the panel, the riser tubes were frequently soldered or welded directly to the panel. 
However, thermal expansion problems were also encountered with these prior devices.  Thus, when the device was placed in the sun, the solar collector not only absorbed heat for transfer to the fluid medium, the heat also caused the panels and riser tubes to expand.  The expansion problem was particularly vexatious because the panel and the riser tubes were generally of different thicknesses and, even when made of the same material, expanded at different rates causing stress upon and eventually rupturing the bond between the panel and the riser tubes.  Once the bond was broken, the thermal conduction between the panel and the associated riser tubes was no longer efficient and, frequently, was ineffective. 
In order to overcome this problem, the prior art made several attempts to develop alternative means for connecting the riser tubes to the solar collector panels.  For example, several describe the use of a plurality of spring or elastic clips placed at a plurality of locations along the length of a riser tube to secure the collector panel attached thereto. ( See: for example, Marles et al.,  U.S. Pat.  No.  4,164,935; Sorenson et al.,  U.S. Pat.  No.  4,094,301; and Ligier, France, Publication No.  2530788).  These proposed solutions generally involved very short clips and required the use of many clips which were placed in spaced generally parallel relationship to each other along the axial length of a riser tube.  As disclosed, the clips did not cover the complete contact length between the riser tube and the collector panel and therefore did not maximize the heat conductivity therebetween.  Further, the materials used for these various clips was not specifically named in all these references, although Marles did teach that cadmium-plated steel was the material of choice.  Further, these references abandoned the single large collector panels theretofore commonplace and taught the use of a plurality of smaller panels intermediate of and connected to each riser tube. 
In another prior art teaching, seeking to enhance the heat exchange relationship of the solar panel to the riser tubes, Cotsworth et al.,  U.S. Pat.  No.  4,120,284, discloses a C-shaped resilient metal clip.  The mouth of this clip is made to spring outward as it is pressed onto the riser tube and then resiliently return to normal after circumscribing the tube.  In practice, the collector panel is first wrapped around the riser tube and then is secured thereto in heat conducting relationship therewith by the application of the clips.  Cotsworth suggests that a plurality of clips may be disposed along the axial length of each riser tube in spaced relationship to each other, or, alternatively, a single elongated clip may be employed to engage the collector sheet over the full length of the riser tube.  This clip is designed for use with a single collector panel curved around several riser tubes.  The clips are made from a resilient metal. 
Finally, McCall, U.S. Pat.  No.  4,624,242, teaches a connecting member, preferably formed of extruded aluminum, for receiving conduits partially wrapped with thin metal heat exchange fins or panels, preferably aluminum.  The connecting members have two oppositely facing conduit receiving portions and are formed so inner diameter (I.D.) of the conduit receiving portion conforms to the outside diameter (O.D.) of the conduit received therewithin.  The connection is made by a thin metal panel between a conduit and a connecting member.  The conduit is then forced or pressed into the connecting member and the thin metal panel is sandwiched therebetween around the conduit to conform to the curvatures of the conduit and the connecting member.  In this way, the thin metal sheet and the conduit are securely held inside the connecting members in thermal conducting relationship. 
While most of the cited references suggest the use of thermally conductive materials, such as copper and aluminum, for the riser tubes, none recognized the existence of the major problem created by the cyclic elongation and retraction of the riser tubes upon their end connections and the ruptures and leaks which resulted.  It is toward the solution of that problem that the present invention is directed. 
The present invention thus involves the utilization of cambered shapes of and the use of connection members and riser tubing deliberately made from materials having different relative thermal expansion properties that coact to eliminate the compressive and tensile stresses which heretofore caused failure and leakage when the uncontrolled thermal expansion and contraction of the tube, caused the fluid connectors to fail.