An evacuated-tube collector by
Canadian Solar Technologies, contains several individual glass tubes,
each containing an absorber plate bonded to a heat pipe and suspended in
a vacuum. The pipe transfers the heat efficiently to a condenser
through the top of the tube. The condensers are clamped to heat
exchange blocks in a well-insulated manifold. A row of connected air-less glass tubes heat water through an evaporation and condensation process.
SEIDO EVACUATED TUBES were developed by Bejing Sunda and Daimler-Benz
Aerospace. Over ten years of thorough
research and development resulted in one of the most efficient
designs in the industry. 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 aluminum nitride selective coating to ensure high absorption and
high efficiencies even on cloudy days
and under low temperatures.
Heat Pipe Technology
Heat 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. Sunda's SEIDO 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.
Advantages of the Technology
High heat transmission rate
Fast Start-up
One-way heat conduction
The homogeneous heat distributing on the surface of the condenser
Selective Coating
The absorber is treated with an aluminum-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 and
reduces less than 8% heat loss.
Advantages of the Technology
High absorbency to guaranty the effective heat from solar irradiation
Low emittency against the heat loss by heat emission
Evacuated Tube Technology
Heat pipes are inserted into the aluminum 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. Sunda uses 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.
Advantages of the Technology
Effective hermeticity
Resistance in harsh environments, e.g. against corrosive materials, vibrations, serious temperature fluctuations
Long-term, reliable protection of the packaged component --------------------------------------------------------------------------
