U.S. Pat. No. 6,014,968 entitled “Tubular Heating-Pipe Solar Water-Heating-System With Integral Tank” that issued Jan. 18, 2000, on a patent application filed in the name of Siang Teik Teoh (“the '968 patent”) discloses a solar water-heating system having collector core that includes a plurality of hollow heating-pipes. Each heating-pipe has a longitudinal axis and an interior that is surrounded by an outer wall. When assembled into the collector core of the solar water-heating system, the heating-pipes are aligned substantially parallel to each other, and in use are adapted to be inclined to the horizontal. Thus, when in use each of the heating-pipes has an open upper end that is elevated above the heating-pipe's closed lower end. The open upper end of the heating-pipes disclosed in the '968 patent extend directly to, open into, and communicate directly with:                1. a lower level of a hot-water storage-tank; or        2. a lower portion of another the solar water-heating-panel.        
Each of the hollow heating-pipes has an outer wall that surrounds a hollow cooler-water return-pipe. The cooler-water return-pipe within each of the heating-pipes has an internal cross-sectional area that is approximately equal to one-third (⅓) to one-half (½) of an internal cross-sectional area enclosed by the surrounding heating-pipe's outer wall. The cooler-water return-pipe within each of the heating-pipes also has a length that is slightly longer than a length of the outer wall of the heating-pipe. Thus, the open upper end of the cooler-water return-pipe extends beyond the open upper end of the heating-pipe's outer wall. In this way the upper end of the cooler-water return-pipe extends into and communicates directly with:                1. the lower level of a hot-water storage-tank; or        2. the lower portion of another the solar water-heating-panel.A lower end of each cooler-water return-pipe is perforated so fluid may flow outward from within the lower end of each cooler-water return-pipe toward the surrounding outer wall of the heating-pipe.        
Preferably, the solar water-heating system disclosed in the '968 patent includes a transparent cover, usually made of glass, that is disposed immediately adjacent to and shields the heating-pipes. The solar water-heating system disclosed in the '968 patent when assembled with evacuated glass thermosyphon coaxial heating tubes exhibits the highest thermal efficiencies of all presently known solar water-heating systems.
Existing solar hot water panels are susceptible to mechanical damage if water in the heating-pipes freezes and cracks the heating-pipes. Prior systems have addressed this problem through an indirect system in which solar radiation heats an antifreeze solution in heating-pipes, or is heated at a condenser portion of heat pipes. The hot antifreeze solution then circulates through a heat exchanger located in the hot-water storage-tank to heat the water. This type of solar water-heating-system is inefficient since the water is only indirectly heated by the antifreeze solution. Consequently, this indirect solar water-heating-panel system heats less water than a direct solar water-heating-system in which the water being heated circulates through heating-pipes. Moreover, indirect solar water-heating panel systems are more expensive and complicated than direct solar water-heating-systems, and require maintenance including regular topping up of any intermediate-working liquid antifreeze solution if such is used.
Another problem sometimes experienced with prior water filled solar hot water panels occurs if the hot-water storage-tank's and heating-pipes' water supply is accidentally interrupted for a few weeks. During the water supply interruption the hot-water storage tank and heating-pipes can boil dry. After the hot-water storage-tank and heating-pipes boil dry, restoring the water supply to the solar hot water panel on a hot day introduces cold water swiftly into hot heating-pipes. Swift introduction of cold water into hot heating-pipes can cause the heating-pipes to bend to such an extent that they can shatter either:                1. a glass tube of an evacuated thermosyphon coaxial heating tube that surrounds the solar hot water panel's outer wall and hollow cooler-water return-pipe; or        2. an immediately adjacent transparent glass cover.