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The evacuated tube collectors are an entirely
different approach to solar water heating. Instead
of many water filled copper pipes, these collectors
use multiple vacuum filled glass tubes, each with a
tiny amount of antifreeze hermetically sealed within
a small central copper pipe. When heated by the sun,
this antifreeze converts to steam, rises to the top
of the tube, transfers its heat to a collector
header, then condenses back into liquid and repeats
the process.
Because heat doesn't easily transfer through a
vacuum, 92% of the thermal energy hitting the
absorber plate stays within the evacuated tube and
passes to the collector header. This is a huge
advantage because a standard flat plate collector
radiates much of its accumulated heat to the
surrounding atmosphere like any other hot object.
The evacuated tubes are also completely modular.
Although rarely necessary, one or more tubes can be
removed and replaced without affecting the other
tubes in the array. There is no actual liquid
transferred from the evacuated tube to the collector
header...just heat. Evacuated tubes also start
absorbing heat earlier in the day than flat plates
due to their convex design and the tiny amount of
antifreeze within the tube is freeze protected down
to -50 degrees below zero.
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A
radiant heating system is considered "open" any time
the same hot water is used for both heating and
domestic hot water. This type of system is very
efficient because a single heat source (in this
case, solar with a fossil fuel back-up) provides for
all the home's hot water needs. In other words, the
homeowner doesn't need two completely separate
systems, many times with overlapping mechanical
components, performing separate heating tasks.
Why use solar energy for domestic hot water and
heating? Well, besides the obvious advantages of
using an unlimited, free (once system components are
paid back), renewable heat source, solar water
heaters interface well with radiantly heated floors
because the large thermal mass common to radiant
systems provides an excellent storage medium for the
energy generated during the day. At night, this
stored thermal energy is slowly released into the
living space and a steady, even, and consistent
comfort level is maintained.
The following schematic illustrates the components
necessary for an open system that uses the external
heat exchanger and antifreeze approach.
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