I was just at a solar thermal workshop for the Heliodyne commercial pump stations. It was quite good. What stuck out to me was how Heliodyne recommends pressurizing a solar thermal system to protect the propylene glycol.
First, let me explain. Propylene Glycol is used in solar thermal systems when they need protection from freezing. Mostly this is used in closed loop systems (though not exclusively) in parts of the world where the temperature commmonly gets below freezing. Propylene glycol is related to Ethylene glycol. Ethylene glycol is what is used in most vehicles to keep the engine from freezing. Ethylene glycol is highly toxic. Propylene glycol is considered non-toxic (though I wouldn’t recommend drinking it!). If you look in your fridge and medicine cabinet, I guarantee that you will find a product with propylene glycol as one of the ingredients. You will frequently find it in; candy bars, ice creams, shampoos, conditioners, toothpaste (the list goes on and on).
In a solar system the propylene glycol keeps the pipes and collectors from freezing. However, most propylene glycols break down at a temperature less than the temperature that the solar collectors produce. This is especially true during the summertime when solar systems produce their most heat and can have their lightest load. Also known as ‘Stagnation Temperature’. Stagnation Temperature is the temperature that the collector reaches in full sunlight when the solar system is not operating. These temperatures can be 320F and higher, depending on the collector type. The max. recommended temperature of Dowfrost propylene glycol is 250F. This is important because propylene glycol begins to break down at temperatures above and near 250F. When the glycol breaks down, it becomes more acidic. If this happens enough, the propylene glycol can eat through all of the collector and solar system piping! To extend the life of the propylene glycol, the manufacturers add inhibitors to coat and protect the piping, collectors and heat exchangers. They also add buffers to increase and maintain the alkalinity of the propylene glycol.
It’s important to know these basics to understand the two trends promoted by manufacturer’s of closed loop solar thermal systems. They both propose that their way is the best to prolong the life of the propylene glycol. I’m not certain yet which one is best.
Heliodyne runs their systems at higher pressure’s than all other solar thermal system manufacturer’s. They do this so that as a solar system heats toward the stagnation temperature, the system pressure rises enough to keep the propylene glycol from boiling. This way the solar system never boils, so that the propylene glycol does not go through the stress of that phase change and will last longer.
Schott and many other professionals use what is known as ‘steam back’ to protect the glycol. With steam-back you set up the solar system expansion so that the propylene glycol vaporizes (boils) at a temperature less than 250F. Since the vaporization pushes all of the glycol out of the collectors, the propylene glycol cannot overheat and is thus protected from breaking down prematurely.
I have been a subscriber to the steam-back method. However, Heliodyne may have a solution also. What is needed is for someone to do testing on the modes of breakdown of propylene glycol. That’s a study I would like to see!