before being accumulated in the circulating liquid. A flat plate collector making use of a honeycomb structure to reduce heat loss also at the glass side too has also been made available commercially. Most flat plate collectors have a life expectancy of over 25 years.. Evacuated tube collectors Evacuated tube collectors are the most common solar thermal technology in the world. They make use of a glass tube to surround the absorber with high vacuum and effectively resist atmospheric pressure. The vacuum that surrounds the absorber greatly reduces convection and conduction heat loss, therefore achieving greater energy conversion efficiency. The absorber can be either metallic as in the case of flat plate collectors or being a second concentric glass tube ("Sydney Tube"). Heat transfer fluid can flow in and out of each tube or being in contact with a heat pipe reaching inside the tube. For the latter, heat pipes transfer heat to the fluid in a heat exchanger called a "manifold" placed transversely with respect to the tubes. The manifold is wrapped in insulation (glass wool) and covered by a protective metal or plastic case also used for fixing to supports. Glass-metal evacuated tubes are made with flat or curved metal absorber sheets same as those of flat plates. These sheets are joined to pipes or heat pipes to make "fins" and placed inside a single borosilicate glass tube. An anti-reflective coating can be deposited on the inner and outer surfaces of such tubes to improve transparency. Both selective and anti-reflective coating (inner tube surface) will not degrade until the vacuum is lost. A high vacuum-tight glass-metal seal is however required at one or both sides of each evacuated tube. This seal is cycled between ambient and fluid temperature each day of collector operation and might lead to failures in time. Glass-glass evacuated tubes are made with two borosilicate glass tubes fused together at one or both ends (similar a vacuum bottle or dewar flask). The absorber fin is placed inside the inner tube at atmospheric pressure. Glass-glass tubes have a very reliable seal, but the two layers of glass reduce the amount of sunlight that reaches the absorber. The selective coating can be deposited on the inner borosilicate tube (high vacuum side) to avoid this, but heat has then to flow through the poorly conducting glass thickness of the inner tube in this case. Moreover, moisture may enter