Solar Drainback Systems (Part 1)
After 30 years of designing and installing hydronic systems, I’ve developed a philosophical statement about antifreeze solutions:
“The only good thing about antifreeze is that it doesn’t freeze.”
Although these words certainly don’t rank up there with other notable quotes, they none-the-less suggest that use of antifreeze in hydronic systems has some “baggage” associated with it. Here’s a quick summary:
So, if antifreeze has so many issues, why is it used in the majority of solar thermal DHW systems? Opinions vary on this, but I think it’s because this approach allows installation of the piping between the collectors and storage tank to take any available route (up, down, around, under, over, etc). Since the fluid remains in the circuit, there is no need to pitch the piping.
The AlternativeAll solar thermal systems installed in North America must be protected against freezing. Aside from using antifreeze, the answer is to drain all water from the collectors and any piping installed in non-heated space whenever the solar collection process is not operating.
There is more than one way to do this. One approach that had its chance in the evolution of solar thermal design is called a “draindown” system. A schematic of the concept is shown in Figure 1.
The concept is simple: The collectors, piping, and tank all contain pressurized domestic water. When a temperature sensor detects a near-freezing condition at the bottom of the collector, two solenoid valves close to isolate the storage tank from the collectors and exposed piping. A third solenoid valve opens to allow the water in the collectors and exposed piping to drain out. A vacuum breaker at the top of the collector array allows air in to expedite drainage.
Most of the time all of this works as it’s supposed to. Unfortunately “most of the time” doesn’t count when it comes to protecting several thousands of dollars worth of installed hardware from freeze damage. If one of the valves sticks, or a freeze-detecting sensor drifts out of calibration, the result can be a costly hard freeze.
There is also concern about scaling or corrosion caused by fresh potable water in the collector circuit. The bottom line: The solar thermal industry made a diligent effort at making this concept work many years ago, and has since conceded that it just presents too many possible failure modes. I strongly concur and advise against this approach.
Drainback SystemsGravity is a very reliable phenomenon, more reliable than solenoid valves and temperature sensors. Why not put it to work in ensuring that water exits a solar collector array whenever the system is not operating? That’s exactly what a drainback system does. When the collector circulator shuts off, the water in the collector array and exposed piping immediately drains back to a reservoir within heated space. No valves, no freeze sensors, no vacuum breakers needed. The fundamental concept is shown in Figure 2.
When the differential temperature control determines it’s time to collect heat, it turns on the collector circulator(s). In some drainback systems, this is a single “high head” circulator. Other drainback systems use two circulators mounted in series. In either case, the circulator(s) must be capable of lifting water from the static water level (shown by the dashed green line), all the way to the top of the collector array.
What happens next has never ceased to amaze me: With sufficient flow velocity, the water passing over the top of the piping circuit begins to entrain air and drag it back down toward the storage tank. Eventually, all the air in the return pipe is pushed/dragged back to the drainback space at the top of the storage tank. The filled return riser acts as a siphon, and essentially cancels out the lift head. The system then operates very similar to a closed- loop/fluid-filled system. In systems with two circulators it’s possible to turn off one of those circulators once the siphon is established (more on this later).
One of the chief advantages of drainback systems is that no antifreeze is required. This not only eliminates the cost of the fluid, but also that associated with a heat dump provision to protect the fluid during collector stagnation.
When stagnation conditions occur there is no water in the collectors. I call this “dry stagnation.” All collectors meeting the current OG-100 rating from the Solar Rating and Certification Corp. must pass a stagnation test to prove they can survive such conditions.
Some drainback systems also eliminate the need for heat exchangers between the collector and the water in the storage tank. The same water that passes through the space heating circuits also passes through the collector. This allows for minimum supply water temperature to the collectors and thus maximum efficiency. And this benefit, along with “dry stagnation,” make drainback systems ideal for combi-systems that supply both domestic hot water and some space heating.
There are several specific details required to construct a good drainback system. We are going to get into the specifics in the next two or three Solar Design Notebook columns. So stay tuned to see why drainback solar thermal systems have a lot to offer.