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Evacuated Glass
Solar Thermal Energy Capture Tubes

Typical Configuration

     Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating, which features excellent solar radiation absorption when compared to conventional flat plate collector systems. The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures. This "evacuation" of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes.

A heat pipe is inserted into the length of the tube. The heat pipes contain a special fluid that allows solar radiation to be absorbed and transferred to the stored hot water very efficiently. Solar heat energy gets trapped inside, and cannot escape except via the fluid…. air if you prefer. The solar heating fluid is separate from the water to be heated and is connected to the solar cylinder via a heat exchanger. The maximum temperature reaches 250F-300 F+ in bright sunlight and each tube can boil more than a pound of water per hour, but you would not want to let steam actually form, so about 180 F maximum water delivery temperature (determined by flow rate).

The tubes must be angled at a specific degree above horizontal so that the process of vaporizing and condensing functions. There are two types of collector connection to the solar circulation system. Either the heat exchanger extends directly into the manifold ("wet connection") or it is connected to the manifold by a heat-conducting material ("dry connection"). A "dry connection" allows to exchange individual tubes without emptying the entire system of its fluid.

Evacuated tubes offer the advantage that they work efficiently with high absorber temperatures and with low radiation. Higher temperatures also may be obtained for applications such as hot water heating, steam production, refrigeration, and air conditioning. Evacuated tubes can still work efficiently even on cloudy days, because it is able to absorb the energy from infrared rays passed through clouds.

However, evacuated-tube collectors are more expensive than flat-plate collectors, with unit area costs about twice that of flat-plate collectors.

The collectors are usually made of parallel rows of transparent glass tubes. The tubes are like a 4-foot long thermos bottle with vacuum between two outer tubes. There is a purple looking solar energy adsorption selective coating on the middle tube, so solar heat energy gets trapped inside, and cannot escape except VIA the water (or other working fluid…. air if you prefer) flow in and around the center tube.

The tubes mount on 4-inch centers and are 2 inches OD.
They are often combined with multiple tubes to form a solar array. Combined with manifolds, this is an inexpensive way to capture a lot of solar energy to make domestic hot water or heat for the home.

Evacuated tube collectors work even in the coldest climates!

The solar capture tubes offered by Fossil Freedom were made by Owings-Illinois Glass Company, for Sunpac.

The tubes are made of extremely strong borosilicate glass.

These cost Uncle Sam $250.00 per tube new.

These solar capture tubes were working well, when removed from a large hot water system in order to add on to an existing building. Each tube is re-tested before sale.

Manifolds are not included, but we will provide plans for do-it yourself manifolds, using standard plumbing fittings that can be found at any good home improvement store, or we can provide tube-ready manifolds at a very reasonable cost.

          Now available at the Fossil Freedom Store



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