What is an evacuated tube?

Evacuated tubes are the absorber of the
solar water heater. They absorb solar energy
converting it into heat for use in
water heating. Evacuated tubes have already
been used for years in Germany, Canada, China,
and the UK. There are several types of
evacuated tubes in use in the solar industry.
Evacuated tube collectors use the most
common "twin-glass tube". This type of tube is chosen for its reliability, performance and low manufacturing cost.

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 (Al-N/Al) which features excellent solar radiation absorption and minimal reflection properties. 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 gases forms a vacuum, which is an important factor in the performance of the evacuated tubes.

Why a vacuum?
As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, we don't want to
lose it!! The vacuum helps to achieve this. The insulation properties are so good that while the inside of the tube may be 150C / 304F , the outer tube is cold to touch. This means evacuated tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-T conditions).

In order to maintain the vacuum between the two glass layers, a
barium getter is used (the same as in television tubes). During
manufacturing of the evacuated tube this getter is exposed to
high temperatures which causes the bottom of the evacuated
tube to be coated with a pure layer of barium. This barium layer
actively absorbs any CO, CO2, N2, O2, H2O, and H2 out-gassed
from the evacuated tube during storage and operation, thus
helping to maintaining the vacuum. The barium layer also
provides a clear visual indicator of the vacuum status. The silver
colored barium layer will turn white if the vacuum is ever lost. This makes it
easy to determine whether or not a tube is in good condition.

Evacuated tubes are aligned in parallel, the angle
of mounting depends upon the latitude of your location.
In a North-South orientation the tubes can passively
track heat from the sun all day. In an East-West
orientation they can track the sun all year round.

The efficiency of an evacuated water heater is dependent upon a number of factors, one important one being the level of evacuated radiation (isolation) in your region.

Evacuated Tube Basic Specifications

The operation of the solar collector is very simple.

1. Solar Absorption: Solar radiation is absorbed by the evacuated tubes and converted into heat.
2. Solar Heat Transfer: Heat pipes conduct the heat from within the solar tube up to the header.
3. Solar Energy Storage: Water is circulated through the header, via intermittent pump cycling. Each time the water circulates through the header the temperature is raised by 5-10C / 9-18F. Throughout the day, the water in the storage tank is gradually heated.