Green Building Elements: Evacuated Solar Tubes

June 4, 2007

Photo Credit: Silicon Solar Inc.We are pretty familiar with most of the ways solar energy is collected. There are photovoltaic panels (PV) which directly convert sunlight into electricity. Solar hot water systems are another widely known system. Water circulates through a series of tubes or through a pipe to be heated by sunlight. And solar concentrators use mirrors to focus sunlight on a narrow area, either for direct heating, or to boil water to make steam for electrical generating purposes.

Evacuated tube heaters are another method of collecting solar energy. Rather than running the water through a long circuitous course, each tube is a separate heat collector. It is made of a clear glass cylinder which allows sunlight to pass through, and a central heat collector tube. The evacuated tube insulates the collector element, which makes it more efficient in colder environments. The collector itself is typically filled with an antifreeze mix rather than just water. The top of the tube has a heat exchange element which is prevents contamination of the water being heated. The tubes are collected together in an array, with a manifold across the tops, containing the heat exchangers.

Image credit: Enviro-friendly.comEvacuated tube collectors are more efficent than water circulating collectors, and can reach higher temperatures, especially in wintertime. Because of their greater efficiency, a water heating system using evacuated tube solar collectors will generally be smaller than a comparable water circulating assembly. The cylindrical form of the tubes also means that the collectors are always perpindicular to the sun, while flat collectors lose more efficiency early in the morning and late in the afternoon unless they are rotated to track the sun (see solar energy diagram). Evacuated tube systems can also be easier to operate in a cold environment where concerns about the system freezing at night, particularly during cold winter months, is an issue.

Evacuated tube collectors also help to minimize maintenance. The collectors themselves are sealed, and the water system only has brief contact with the heat exchangers in the manifold. In the event of damage, individual tubes can be fairly readily swapped out, whereas holes in tubing and broken glass over a flat array can be harder and more expensive to repair.

There are a few tradeoffs with evacuated tube collectors. They tend to be more expensive. They can be broken (by hail or falling branches from nearby trees), which looses the vacuum and the efficiency of the tube, and necessitates eventual replacement. They are also sometimes unsuitable for snowy climates, as well, because the heat gathering element is insulated with a vacuum which means that the tubes themselves will stay cool, and can remain covered with snow instead of melting it off as the collector gets heated up.

In the right location, evacuated tube solar collectors can be a good choice for use in conjunction with a hot water system or a radiant floor system (or both). Any form of solar hot water system has a much shorter payback period than a photovoltaic system, and is well worth considering. Evacuated tube collectors are not as well known, but should be part of that evaluation, as well.

Links:

Silicon Solar

Sunpower Canada

Enviro Friendly Products

Wikipedia

 


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