evacuated tube collector
An evacuated tube collector is a type of solar collector that
can achieve high temperatures, in the range 170°F (77°C) to 350°F
(177°C) and can, under the right set of circumstances, work very efficiently.
Evacuated-tube collectors are, however, quite expensive, with unit area
costs typically about twice that of flat-plate
collectors. They are well-suited to commercial and industrial heating
applications and also for cooling applications (by regenerating refrigeration
cycles). They can also be an effective alternative to flat-plate collectors
for domestic space heating, especially in regions where it is often cloudy.
For domestic hot water heating, flat-plate collectors tend to offer a cheaper
and more reliable option.
An evacuated-tube collector consists of parallel rows of glass tubes connected
to a header pipe. Each tube has the air removed from it to eliminate heat
loss through convection and radiation. Evacuated-tube collectors fall into
two main groups.
Direct-flow evacuated-tube collectors
These consist of a group of glass tubes inside each of which is a flat or
curved aluminium fin attached to a metal (usually copper) or glass absorber pipe. The fin is covered with a selective coating that absorbs solar radiation
well but inhibits radiative heat loss. The heat transfer fluid is water
and circulates through the pipes, one for inlet fluid and the other for
outlet fluid. Direct-flow evacuated tube collectors come in several varieties
distinguished by the arrangement of these pipes.
- Concentric fluid inlet and outlet (glass-metal). These use a single
glass tube. Inside this is a copper heat pipe or water flow pipe with
attached fin. This type of construction means that each single pipe
can be easily rotated to allow the absorber fin to be at the desired
tilt angle even if the collector is mounted horizontally. The glass-metal
design is efficient but can suffer reliability problems. The different
heat expansion rates of the glass and metal tubes can cause the seal
between them to weaken and fail, resulting in a loss of vacuum. Without
a vacuum, the efficiency of an evacuated-tube collector is no better,
and may be worse than, that of a flat-plate collector.
- Separated inlet and outlet pipes (glass-metal). This is the traditional
type of evacuated-tube collector. The absorber may be flat or curved.
As in the case of the concentric tube design, the efficiency can be
very high, especially at relatively low working temperatures. The weakness
again is the potential loss of vacuum after a few years of operation.
- Two glass tubes fused together at one end (glass-glass). The inner
tube is coated with an integrated cylindrical metal absorber. Glass-glass
tubes are not generally as efficient as glass-metal tubes but are cheaper
and tend to be more reliable. For very high temperature applications,
glass-glass tubes can actually be more efficient than their glass-metal
Heat pipe evacuated-tube collectors
These consist of a metal (copper) heat pipe, to which is attached a black
copper absorber plate, inside a vacuum-sealed solar tube. The heat pipe
is hollow and the space inside, like that of the solar tube, is evacuated.
The reason for evacuating the heat pipe, however, is not insulation but
to promote a change of state of the liquid it contains. Inside the heat
pipe is a small quantity of liquid, such as alcohol or purified water plus
special additives. The vacuum enables the liquid to boil (i.e. turn from
liquid to vapor) at a much lower temperature than it would at normal atmospheric
pressure. When solar radiation falls the surface of the absorber, the liquid
within the heat tube quickly turns to hot vapor rises to the top of the
pipe. Water, or glycol, flows through a manifold and picks up the heat,
while the fluid in the heat pipe condenses and flows back down the tube
for the process to be repeated.
An advantage of heat pipes over direct-flow evacuated-tubes is the "dry"
connection between the absorber and the header, which makes installation
easier and also means that individual tubes can be exchanged without emptying
the entire system of its fluid.
Some heat pipe collectors are also supplied with a built in overheat protection
– when a programmed temperature has been reached, a "memory metal"
spring expands and pushes a plug against the neck of the heat pipe. This
blocks the return of the condensed fluid and stops the heat transfer.
A drawback of heat pipe collectors is that they must be mounted with a minimum
tilt angle of around 25° in order to allow the internal fluid of the
heat pipe to return to the hot absorber.
Evacuated-tube collectors get much hotter than flat-plate collectors. The
high temperatures they produce, which can exceed the boiling point of water,
can cause significant problems in a domestic solar water heating or solar
space heating system. It's therefore crucial to make sure there is always
an adequate load on the system to keep the temperatures below 100° (212°F).
The glass tubes are fragile, especially so since they are made of annealed
glass, which is much more delicate than tempered glass. Care must be taken
when transporting and handling the glass tubes.
Finally, evacuated-tube collectors, unlike flat-plate collectors (the surface
of which is always warm), do not shed snow. Because the evacuated tubes
are such good insulators, little heat escapes them and the snow that accumulates
on the tubes can stick for a long time. Their surface is also irregular,
so snow packs between the tubes, rendering them ineffective, and the fragility
of the glass tubes makes it impossible to scrape the accumulated snow off.
ENERGY AND POWER