SOLAR DIRECT TECHNOLOGY

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.
 

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.