water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss.

http://www.metaefficient.com/hot-water-systems/solar-tubes-very-efficient-heating.html
Solar Tubes: Very Efficient Heating

by Justin on January 22, 2005


Solar Tubes: Very Efficient Solar HeatingApricus makes a unique solar hot water heater which is 30-40% more efficient than flat plate solar systems.

The secret to the performance is the use of cylindrical evacuated glass tubes to absorb the heat from the sun. Because the tubes are cylindrical, there is always a surface area that is perpendicular to the sun.

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The tubes also house a vacuum between two layers of glass. Why a vacuum? A vacuum is an excellent insulator. The insulation properties are so good that while the inside of the tube may be 304°F (150°C), the outer tube is cold to touch. This means that solar tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss.

More information: Apricus