Knowing the limits: Siphon limitations in drainback systems

摘要

When it comes to freeze protection for solar thermal systems, my preference is a drainback configuration, operating with water only. With proper design and installation, the water in the solar collectors and their associated piping simply drains back into the storage tank, or a separate "drainback" tank, whenever the collector circulator turns off. I know this works, and works well, because I've lived with such, a system for more than three decades in Upstate New York. Figure 1 shows a closed-loop drainback-protected system, designed around a pressure-rated thermal storage tank. In this system, the air space for holding the collector drainback water, and for accommodating thermal expansion of system water, is at the top of the thermal storage tank. Closed-loop drainback systems also can use a separate drainback/expansion tank installed above a completely filled thermal storage tank. This will be discussed later.