A drainback or a closed loop solar thermal system is still a huge debate in the solar thermal industry. I have always maintained that each system is right for the right application. Drainback or Closed Loop systems are just different ways to ensure that the solar system does not freeze. A closed loop system uses Propylene Glycol mixed with water (typically about 50%) to keep the solar system from freezing. A drainback system simply drains back all of the fluid in the solar system to a collection tank when the system is not running. With the fluid out of the collectors and exposed piping, there is nothing to freeze.
For the bigger systems (commercial and industrial), a drainback system rarely makes sense. Drainback systems make the most sense for small residential to large residential (or light commercial) systems. The best benefit of a drainback solar thermal system is that it can simply shut down when the system has no place to put the heat (ie, the solar storage tank has been charged and can take no more heat). Of course, this is assuming that the collectors can also handle stagnation conditions, many cannot, especially evacuated tube collectors. Since the drainback system can handle staganation, this makes them ideal for applications where the system may be oversized during some periods of the year (like a heating system in Colorado). The drainback system also is a simpler system to maintain, with only water as the fluid (though you can added propylene glycol for added safety). The downsides of drainback systems; A large pump is required to overcome push fluid all of the way up and into the collectors, so the pump uses more power than a closed loop system. The plumbing and installation of the collectors is critical and any mistakes can destroy the whole system by freezing. This makes the systems typically more difficult to install and usually only installed by solar contractors with more experience.
Closed loop systems are much more flexible and easily adapted to large commercial and industrial applications. They also can be used for residential systems. With closed loop solar thermal systems, the biggest issue is around stagnation. Stagnation occurs when the solar system doesn’t have anywhere to put the heat that is being collected from the sun. This is when the flat plate systems can get over 340F and the evacuated tubes as much as 400F. At these temperatures there are several issues. The first is that the propylene glycol becomes more acidic the longer the fluid is at the higher temperatures. The second has to do with the fluid boiling. This is more of an engineering problem. The system has to be designed to accommodate boiling or it will overpressurize and release fluid (also known as a ‘blow-down), which can be dangerous. See my post about glycol (Glycol in Solar Thermal Systems….) In general, with glycol systems they should only be used in applications where the solar fraction is low. No higher than 50% is preferred, especially in commercial systems. The more consistent the year round load on the system, the better it will perform, and the less the system has the possibility of needing frequent maintenance to change out the fluid.
To recap; Use the right system for the right application. Closed loop solar thermal systems are best suited for applications that have a consistent year-round load. Drainback solar thermal systems are best suited for systems that must have a higher solar fraction and or have a load that is not constant. If you want to read more about me or my company follow this link www.simplyeff.com