The Development of Drain Back Systems
In an effort to overcome the many problems of glycol systems, early researchers turned to other methods.
To overcome boiling and pressure problems with glycols, high temperature silicon oils were used. Unfortunately, they were very expensive, had poor heat transfer characteristics, and tended to leak out of soldered joints.
Others tried air as the heat transfer medium. It won’t boil or freeze. Blowers and ductwork to the collectors were a problem, and storing the heat from the air in a pile of rocks brought its own problems of mold and dust. You can’t fab a rock pile and ship it to a site.
Others went back to plain water as the heat transfer fluid. It has the highest heat transfer capacity of any fluid. All others are measured against water, which is rated as 100%. Glycol is about 85%; silicon oil is about 20% as good as water.
Since water will freeze and boil, the idea is to drain the water from the collectors at night, or when a high temperature limit is reached, so it is not there when the extreme conditions come. The system doesn’t have to be pressurized, so tanks don’t have to have an ASME pressure rating, which can double or triple the price. Non pressurized systems don’t need pressure relief valves and expansion tanks.
Early designs included air vents at the high points and heat exchangers between the collectors and storage. Some thought a vacuum breaker was required at the top to make the water drain out when the pump stopped. Some even installed a pipe between the collector supply and return lines with an electric valve to guide all the water to the return line for draining.
All these vestiges of glycol systems only caused problems. Air vents and vacuum breakers introduce fresh oxygen into the water, accelerating corrosion. Conventional air vents on tanks cause evaporation losses, which required periodic refilling (and fresh oxygen). Protecting against corrosion by lining the tank is cost prohibitive above a certain size, and subject to cracking during transport. Check valves only complicate draining the water from the collectors.
There was still room for improvement…