ASK THE EXPERTS: Drainback Pumps

Intermediate
Drainback tank in attic to decrease pumping head.

Thanks for James Dontje’s article about his home drainback solar water heating system (“Designing a PV-Powered Drainback Solar Hot Water System” in HP120). Along with that information, Gary Reysa’s affordable drainback systems at builditsolar.com have inspired us to implement a similar project at our home.

Our grid power has been reliable, but it’s getting pricier, and we also recognize its myriad externalized costs. Before we install the solar thermal system, we’re planning to install a PV system with battery backup to cover crucial loads (2 kWh per day) and reduce our fossil-fuel footprint.

One of those crucial loads would be the drainback pump, which would likely need to be AC to handle 25 feet of head. Could you recommend an energy-efficient variable-speed or low-flow AC pump that would meet this need? Also, how could we estimate the watt-hours needed to run an AC pump with 25 feet of head? Or should we consider piggybacked DC pumps to get the job done?

Jonathan Bentley • Bryson City, North Carolina

To make a lower-power (and lower-cost) drainback system, one strategy is to strategically locate your drainback tank to reduce the pump’s head requirements. The system in the article served a two-story house—the hot water storage tank was a little below first-floor level, in the garage, and the drainback tank was suspended from the ceiling in a second-floor bathroom, which reduced the pump head by more than half. The pump was located at the level of the storage tank in the garage. The pump needs to be low enough that it can never pull in air when the drainback tank fluid is low. At a minimum, it needs to be at least a couple of feet below the bottom of the drainback tank.

Piggybacking either DC or AC pumps is an effective strategy to achieve the required head and to reduce energy use. In a drainback configuration, once the fluid is “over the top” and flowing out of the collector, pumping head requirements drop. The fluid on the down leg creates suction that balances the force needed to lift the water to the collectors. At that point, the pump only has to overcome pipe friction losses. If two pumps are piggy-backed, both pumps are run together for only a minute or so—long enough to get flow established—then the second pump is turned off. A delay timer relay will power the second pump for the user-adjustable time interval before shutting it off and letting the main pump carry the load.

Measuring the pumping energy required is a little tricky. Pumps have a rated power, but depending on the configuration, may use a bit less power than their rating. For sizing your power source, use the rating, but remember that there will be a current surge at startup. Your PV system’s inverter will need to handle this surge. Most inverters can operate at double their rating for a short period of time—long enough for a motor to start up—but you’ll need to verify this specification (usually called “peak” or “surge” power). For overall daily energy consumption, multiply the pump’s rating by its run time during a 24-hour period. Pump run time can be hard to estimate for a solar thermal system as it depends on both the amount of sun and the temperature in the storage tank. You can use a location’s average daily sun-hours to estimate the high end of a pump’s energy consumption.

To choose a pump with suitable fluid flow capacity, examine its performance or pump curve, which shows flow versus pressure or head. For example, the pump curve for the Taco 009 pump shows it that can pump 5 gallons per minute at 25 feet of head. Be conservative in your choice—do not ask a pump to perform at its maximum—its “operating point” should be in the middle of the curve.

European energy-efficiency standards have pushed the U.S. market, too, driving the development of efficient pumps with lots of onboard adaptive electronics that can vary pressure and flow. For example, my home heating system (which heats a 2,400-square-foot house) runs on a very efficient 45 W pump. While this pump is a bit underpowered for rapid response, it can deliver enough heat to maintain comfortable indoor temperatures, even at outdoor temperatures of -20°F.

One key pump-choice consideration is whether your drainback loop is sealed or open to the atmosphere. Most circulating pumps are made of cast iron, which can introduce corrosion to the system. As long as the pump loop is sealed, the supply of oxygen is limited and corrosion is low. But with a constantly refreshed oxygen supply in an open-atmosphere situation (for example, a system draining into an unpressurized, open tank), corrosion is a concern. In that case, select a brass or stainless-steel pump housing. If your drainback loop is sealed (and many are), you can stick with the cast-iron pumps.

James Dontje • Johnson Center for Environmental Innovation, Gustavus Adolphus College

Comments (1)

Captron's picture

Hi Jonathan,

This is a great question, something we have been intimately involved in over the last few years as we have optimised our Drainback systems and allowed ease of conversion to off-grid.
We have moved to the latest 24V DC pumps as part of our DSH (Digital Solar Heat) Pump Station revision. These high efficiency pumps have a >30’ head capability, a peak flow rate of 24L/S (~9US/gal/Min) and as mentioned, once you get the flow going, the syphon effect reduces the power required to maintain the flow. If you need only one pump to get the flow going, then the pump runs at low load effect, meaning less energy required.

One reason we changed to the new DC variable speed high tech pumps is their ability to be easily adapted to an off-grid scenario. Another reason is it allows a simple method to power the pumps with a PV panel. 24V, 42W pumps can easily be run directly from a 350W solar panel. These pumps are are brushless and have digital controlled magnetic couplings. The impeller is isolated from the drive system and has no seals.

You have to be careful with PV though. Our research has shown you need to triple the rated pump wattage to achieve a reliable direct power from PV. This seems to be mostly due to the PV panels having poor performance at low angles or off-centre insolation, as compared to say EvT solar thermal panels. All you need is a solar PV panel(s), a 24V battery charger, and preferably 2x12v or 24v batteries.

Good luck.

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