All About Solar Thermal

Starting off let's talk about the types of Solar Thermal Panels.There are a few different types and several different applications. Here they are:

Flat Plate Collector: This panel is a solar collector in a box that is insulated and has a glass or substrate cover. This is called a glazed collector. These can be used in all climates.

Batch Heater: Mostly used in tropical climates where backup water heaters may not be available. These units have storage built into the collector. They can be controlled automatically but mostly they take advantage of the Thermo-syphon process. The hot collector will draw water up to be heated and start a flow.

Evacuated Tube: These collecors typically have a copper pipe inside of a sealed double wall glass tube. They work well when higher temperatures are needed such as steam generation. The name evacuated means that air is drawn out of the tube allowing it to be void of outside influence. The glass tube in direct sunlight will be ambient temperature. Just do not touch the copper tube while it is in the sun.. If you do you will regret it. They also perform better in overcast conditions than flat plate collectors. The downside in my opinion is that because they are not influenced by temperatures outside of the glass tube snow on the collectors melts slower decreasing efficiency during winter months.

Plastic and Rubber Pool Collectors: Solar Pool collectors are made exclusively for this application. Hot water panels ( flat plate or evacuated tube) can be used as well but are more expensive. These panels are unglazed and made of plastic welded parts or extruded in one process. The other is EPDM rubber and usually made in a mat form that rolls out. The plastic panels as a rule have a UV stabilizer formulated in. Not so true with EPDM mat systems. Another factor here is that EPDM rubber turns to gum in chlorene. This can clog filters and produce what is called black pool. This is the staining of liners or marcite.

Whether you are heating water for your home or wanting to heat your pool the first suggestion is to check to see your chosen panel is tested. The place to check is the Florida Solar Energy Center. Just Google FSEC and it will come up.
Now for applications:

Direct Solar Thermal: For domestic hot water systems the potable water flows directly through the panel to the storage tank. This system is used in climates where freeze conditions are highly unlikely. Pool heating is another example because the water is heated directly by the panels.

Drainback: This system does as is suggested and at the end of the day or when Solar gain is not available drains back into a storage reservoir. A pool heating system does the same and drains back into pool itself.

Thermo-syphon System: The batch heater is a good example using no pump or control works simply off of the suns ability to heat water.

Indirect Solar System: It is designed to accommodate climates where freezing weather occurs more frequently and a drain back system cannot be installed. This type of system is also called a Closed Loop system.The solar storage tank and the solar collectors do not differ from those utilized in the direct solar system.
The only difference lies in the way that the water is processed through the solar collector and back to the storage tank. Your potable water is heated indirectly through a heat exchange coil. The solar circulation pump sends a mixture of water and food grade propylene glycol through the solar collector and back to the heat exchange coil.
Heat is transferred to your potable water via the exchange coil which is wrapped around the inner collection tank. The Guardian IP system is freeze proof because the solution in the solar loop is a 2:1 (in colder climates) ratio of water and food grade propylene glycol. The Indirect solar system is only recommended in climates that experience frequent freezing conditions, and where a Drainback system cannot be installed so that it will drain.
Back in the 1990s a manufacturer of Evacuated Tubes used these to make a Solar Thermal Power Plant. The panels created steam that in turn drove a turbine generating electricity for an entire porduction facility. While this was probably not cost effective it did prove that it can be done. To cover all the ways Solar Thermal would take a book. These are the most widely used applications and systems involved.