Vacuum Tube Technology

GreenLand evacuated tube collectorsGreenLand Systems vacuum tubes (evacuated tubes) were developed in Germany by Daimler-Benz Aerospace, with support from UNDP, in 1986. Over ten years of co-operative research and development by Sino-German experts have resulted in today's cutting edge design and superior solar performance.

Ideal for domestic installations, GreenLand Systems vacuum tubes are also readily suited for OEM manufacturers and can be used for various kinds of commerical and industrial applications. The absorber and heat pipe are mounted in a highly stable borosilicate vacuum glass tube. This prevents damage and the heat loss that occurs in conventional solar collectors. The absorber is sputtered with aluminium nitride selective coating to ensure high absorption and highest efficiencies - even on cloudy days.

Heat Pipe Technology

Cross section of systemHeat pipes act like a low-resistance thermal conductor. Due to their thermal-physical properties, their heat transfer rate is thousand's of times greater than that of the best solid heat conductor of the same dimensions.

The GreenLand System vacuum tube heat pipe is a closed system comprised of two meters of copper tubing, an evaporator section, a capillary wick structure, a condenser section and a small amount of vaporizable fluid. The heat pipe employs an evaporating-condensing cycle. The evaporator section is tightly bonded to the absorber plate, where it captures the heat from the absorber and evaporates the liquid to steam, which moves up to the condenser section. The condenser protrudes out from the evacuated tube and is inserted into the heat exchanger manifold. There this steam will be condensed by water flowing through the manifold. Latent heat energy will be released to the process water through this phase change of vapor to liquid.

In vacuum tube solar collectors, the condensation zone is at a higher level than the evaporation zone. The transport medium condenses and returns to the evaporation zone under the influence of gravity. This process is repeated continuously thereby heating the water in the solar loop.

Selective Coating

The absorber is treated with an aluminium-nitride selective coating to achieve highest efficiency of the heat transfer. The coating is applied using a magnetic sputtering technique. This special optical coating transforms more than 92% of the incoming solar irradiation into heat.

Heat Pressure Sealing and Evacuated

Heat pipes are inserted into the aluminium absorbers forming assemblies, which in turn are inserted into the glass tubes. The tubes are made of borosilicate glass which is strong and has a high transmittance for solar irradiation.

In order to reduce the convection heat loss, glass tubes are evacuated to vacuum pressure of <10-5 mbar. GreenLand Systems use a patented technique employing high heat and pressure to insure stable glass-to-metal vacuum seals. In order to keep the stability of the vacuum for a long time, a barium "getter" is used. Through evacuating air out of the glass tube the absorber material and the selective coating are protected from corrosion and other environment influences.

This ensures a lifetime of at least 15 years without loss of efficiency.