Evacuated Solar Tubes
Solar panels are the most common platform in the solar arena, but evacuated solar tubes play an important role as well.
Evacuated Solar Tubes
Evacuated solar tubes absorb the sun’s energy, using it to heat water for solar water heaters. “Evacuated” means that the area between the tubes has all of the air removed and is a vacuum, allowing the most efficient insulation for the inner tube, which contains the liquid to be heated. The pressure proof tubes are made of glass, which has very high insulating values.
In the most common form, the “twin-glass tube”, each tube actually consists of two tubes, one within the other and the heat transfer fluid runs in a countercurrent. An alternative system is a U-tube, where the fluid runs directly through the absorber. The absorber is made of a dark material specialized to collect as much solar energy as possible. A collector attached to the tubes contains a special fluid, which vaporizes at a relatively low temperature. The steam formed then rises to the top of each tube into a heat exchanger and heats up the carrier fluid. The cooled liquid then flows back down, into the pipe, to be heated again. The tubes must be placed at a certain angle for the vaporizing and condensing process to work. The solar circulation system can be attached to the collector in two ways. The first, the “wet” connection, has the heat exchanger going directly into the manifold. The second is the “dry” connection, where the exchanger is connected to the manifold via a substance that conducts the heat from one to the other. The benefit of the dry connection is that the exchange does not necessitate emptying the entire system of its fluid.
Evacuated solar tubes are placed in parallel alignment to form the solar panel, which is mounted at an angle dependent on the latitude of the installation. The panels can be installed with a North – South orientation, giving the most effective absorption on a daily basis, or an East – West orientation, which allows year round flexibility.
Evacuated solar tubes are the most efficient form for heating water with solar power. Traditional flat panels only work at peak efficiency when the sun is directly perpendicular to the panel, so most of the day a flat panel will receive a less efficient angled sun ray. Evacuated Solar tubes are round, so they provide a perpendicular surface to the sun during most hours of sunlight.
An evacuated solar tube system is the most expensive, but it is also the most efficient, producing higher temperature with lower amounts of sunshine. The higher temperatures allow a wider variety of uses than other solar systems, such as steam production and air conditioning.
