WINTER SOLAR

ETC Type
(Evacuated Tube Collector System)

Most vacuum tube collectors in use in middle Europe use heat pipes for their core instead of passing liquid directly through them. Evacuated heat pipe tubes (EHPTs) are composed of multiple evacuated glass tubes each containing an absorber plate fused to a heat pipe. The heat is transferred to the transfer fluid (water or an antifreeze mix typically propylene glycol) of a domestic hot water or hydronic space heating system in a heat exchanger called a "manifold". The manifold is wrapped in insulation and covered by a protective sheet metal or plastic case. The vacuum inside of the evacuated tube collectors have been proven to last more than 25 years, the reflective coating for the design is encapsulated in the vacuum inside of the tube, which will not degrade until the vacuum is lost. The vacuum that surrounds the outside of the tube greatly reduces convection and conduction heat loss, therefore achieving greater efficiency than flat-plate collectors, especially in colder conditions. This advantage is largely lost in warmer climates, except in those cases where very hot water is desirable, e.g., for commercial processes. The high temperatures that can occur may require special design to prevent overheating.

Some evacuated tubes (glass-metal) are made with one layer of glass that fuses to the heat pipe at the upper end and encloses the heat pipe and absorber in the vacuum. Others (glass-glass) are made with a double layer of glass fused together at one or both ends with a vacuum between the layers (like a vacuum bottle or flask), with the absorber and heat pipe contained at normal atmospheric pressure. Glass-glass tubes have a highly reliable vacuum seal, but the two layers of glass reduce the light that reaches the absorber. Moisture may enter the non-evacuated area of the tube and cause absorber corrosion. Glass-metal tubes allow more light to reach the absorber, and protect the absorber and heat pipe from corrosion even if they are made from dissimilar materials (see galvanic corrosion).

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.

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