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Flat-plate collectors usually lose more heat to the environment than evacuated tubes because there is no insulation at the glass side. Evacuated tube collectors intrinsically have a lower absorber to gross area ratio than flat plates because tubes have to be spaced apart. Although several European companies manufacture evacuated tube collectors , the evacuated tube market is dominated by manufacturers in China, with some companies having track records of 15–30 years or more. There is no unambiguous evidence that the two designs differ in long-term reliability. However, evacuated tube technology still needs to demonstrate competitive lifetimes. The modularity of evacuated tubes can be advantageous in terms of extensibility and maintenance, for example, if the vacuum in one heat pipe tube is lost it can be easily be replaced with minimal effort.
In most climates, flat plate collectors will generally be more cost-effective than evacuated tubes. However, evacuated tube collectors are well-suited to cold ambient temperatures and work well in situations of low solar irradiance, providing heat more consistently throughout the year. Unglazed flat plate collectors are the preferred devices for heating swimming pool water. Unglazed collectors may be suitable in tropical or subtropical environments if domestic hot water needs to be heated by less than 20 °C over ambient temperature. Evacuated tube collectors have less aerodynamic drag, which may allow for a simpler installation on roofs in windy locations. The gaps between the tubes may allow for snow to fall through the collector, minimizing the loss of production in some snowy conditions, though the lack of radiated heat from the tubes can also prevent effective shedding of accumulated snow. Flat plate collectors might be easier to clean. Other properties, such as appearance and ease of installation are more subjective and difficult to compare.
Evacuated flat plate solar collectors provide all the advantages of both flat plate and evacuated tube collectors combined together. They surround a large area metal sheet absorber with high vacuum inside a flat envelope made of glass and metal. They offer the highest energy conversion efficiency of any non-concentrating solar thermal collector, but require sophisticated technology for manufacturing. They should not be confused with flat plate collectors featuring low vacuum inside. The first collector making use of high vacuum insulation was developed at CERN, while TVP SOLAR SA of Switzerland was the first company to commercialise Solar Keymark certified collectors in 2012.
Evacuated flat plate solar collectors require both a glass-metal seal to join the glass plate to the rest of the metal envelope and an internal structure to support such plate against atmospheric pressure. The absorber has to be segmented or provided with suitable holes to accommodate such structure. Joining of all parts has to be high vacuum-tight and only materials with low vapour pressure can be used to prevent outgassing. Glass-metal seal technology can be based either on metallized glass or vitrified metal and defines the type of collector. Different from evacuated tube collectors, they make use of non-evaporable getter pumps to keep the internal pressure stable through time. This getter pump technology has the advantage of providing some regeneration in-situ by exposure to sunlight. Evacuated flat plate solar collectors have been studied for solar air condition and compared to compact solar concentrators.