Solar heating is an inexpensive and efficient way to supply all of your hot water needs. Two popular systems that are commercially available through multiple suppliers are the evacuated tube solar collectors and flat panel solar collectors.
The evacuated tube solar collector is defined by a set of vacuum tubes with an absorber and heat pipe suspended within each tube. The advantage of the vacuum tube is that heat loses are minimized through convection and conduction. The tubes are connected at the top by a copper header which connects the collectors to the supply and return piping. Advantages of the evacuated solar tube are that they remain relatively efficient under extreme conditions. They are very modular in design, so if one tube fails, the system can still continue to supply heat and replacing the single tube is simple and inexpensive.
The flat panel solar collector is comprised of an insulated metal box with a glass cover and absorber plate housed within. The glazing is essential to the system performance because the quality of the glazing directly correlates to the amount of heat loss in more extreme temperatures. Pipes move a heat transfer fluid (water or water vapor) along the absorber plate and into a storage tank. Because of the simplicity of the system it is inexpensive, and it has a level of durability and ruggedness that exceeds the vacuum tube system. Aesthetically, it is a low profile system that can blend into a roof by mounting it flush with the roofline.
Each has a proven track record in applications around the country, but which system best suits Boise? The climate region of the Treasure Valley is best characterized by “its semiarid climate with cold, windy winters and warm, dry summers. Winters are very cold, with frequent but short storms alternating with sunny periods.” (Norbert 94). Boise’s unique climate makes it especially challenging for solar collectors to perform efficiently in both cold winters and hot summers.
The two collectors differ in performance under different ambient temperatures. The flat plate collector easily outperforms the vacuum solar tube collector under cold temperatures while the inverse is true in hot temperatures. The efficiency of the flat plate solar collector is less sensitive to the angle that it is mounted whereas the angle that the vacuum tube collector is mounted has a greater effect on its productivity.
One issue that both collectors must address is their ability to control overheating in the hot summer months. While both utilize a dump valve to do so, the flat plate collector dump valve is much cheaper because of the system simplicity while the vacuum solar tube collectors are more costly. New technology is available that provides a thermal spring valve at the head of each vacuum tube that would block the return valve and prevent overheating.
The initial cost to set up a vacuum tube solar collector system is more than a flat panel collector. The advantage of a vacuum tube collector is that each individual tube can be replaced making it relatively inexpensive to maintain and repair the system. The flat panel system is much harder to maintain, and often times when they fail, the entire collector has to be replaced. While all solar collector systems are fundamentally similar, there are inherent differences between vacuum tube and flat panel solar collectors. Due diligence must be taken to ensure the correct system is selected depending on the heating needs of a given project.

SOURCES

Web Article:
• Dr. F. Mahjouri, . "Vacuum Tube Liquid-Vapor (Heat-Pipe) Collectors."www.thermotechs.com. 2005. Thermo Technologies, Web. 16 Sep 2009. <http://thermotechs.com/Downloads/Vacuum%20Tube%20Paper.pdf>.

Publications:
• Lechner, Norbert. Heating, Cooling, and Lighting: Sustainable Design Methods for Architects. 3. Hoboken, NJ: John Wiley & Sons, Inc., 2009.

Lecture:
• Stephan Howe, . "Introduction to Solar Hot Water." Fall 2009 Sustainable Energy Series. Renewable Energy Solutions. Boise. Sept. 10, 2009.