Options with drainback solar hot water
Since the early1980s, solar hot water collection by drainback has undeniably been a reliable, efficient and low maintenance system. Drainback technology has matured to offer sensible design options.
In this article, we will explore innovations to drainback systems that build on the expectations that most solar design builders desire; flexibility, simplicity, longevity and economy. The information about advances in drainback noted here results from speaking with industry specialists throughout North America who have a thorough working knowledge of these systems and, in many cases, have determined that drainback is the optimal choice in solar hot water collection. This opinion is shared by “new” generation solar designers who work with both custom closed loop and drainback system designs.
I work closely with a young design engineer named Russell. He makes it clear to me that drainback is simpler to design, requires less maintenance and is installed at a lower cost. At 26, Russell has five years experience in active solar design, primarily with commercial closed loop systems. He is a sensible designer who understands the advantages and disadvantages of drainback and closed loop systems and prefers to design for drainback when the option is available.
Designed for “fail safe”
Drainback is regarded as a “fail safe” system. The collectors remain empty outside of collection cycles. Empty collectors are protected from freezing water breaks in the cold season or from steam generated by high temperatures of the transfer fluid. Because they are empty, there is no harm to the collectors when the system is not operational.
In contrast, closed loop systems contain a circulated pressurized glycol solution that requires vigilant maintenance. To prevent stagnation of the working fluid, more attention to detail is required. In order for closed loop systems to be fail safe, the glycol would have to be contained or drained if the pumps shut down, and a shutdown would possibly need to be followed up by a service call.
In a closed loop system, the collectors can be installed anywhere in relation to the solar tank or heat transfer. Collectors for drainback, on the other hand, must be mounted above the drainback reservoir, although not necessarily above the solar storage tank. Drainback works well with most roof mounted collectors, since the working fluid can drain back by gravity into the reservoir. To ensure draining by gravity, drainback design should include the specification that there should be no slope ratio on the collectors and that piping be secured dead level to a roof.
It is advisable to add freeze protection (propylene glycol) to the drainback reservoir to prevent the potential freeze that could occur if the array piping sags over time. Dead level installs are often used for rooftops with large collector arrays and large diameter supply and return piping. An excellent example of dead level drainback is the largest domestic hot water system in the state of Wisconsin, located at the Kalahari Resort in Wisconsin Dells. The rooftop is home to 104 vertically-mounted flat plate collectors installed dead level.
Savings in drainback pump first cost and operating costs can be made when selecting the drainback reservoir location. A drainback pump has to lift a column of water from the reservoir to the top of the collectors. The greater the height of the lift, the greater the pump horsepower needed. High lift and greater pump horsepower can be expensive.
Overcoming lift can be an expensive problem when drainback reservoirs are installed in a basement or on a lower floor of a high-rise building. To avoid this problem, the drainback reservoir should be mounted at a high elevation, such as on the top floor. The system pump does not have to be raised with the drainback reservoir but can be installed in the mechanical room with the controls. Installing the reservoir somewhere below the rooftop under the freeze line will allow you to take advantage of smaller pump sizing. I have heard of drainback designs including ridiculously large and very expensive pumps because the engineer did not understand that pump lift requirements are smaller if the reservoir in a tall building is at a high elevation.
Another option for savings is to have two pumps installed on the drainback loop. At start-up, pump 1 energizes to “on” and pump 2 energizes with an “on, time delay to off” sequence. At start-up, both pumps are needed to overcome total lift; to push the air bubble out of the collectors and into the drainback reservoir. Once the air has been displaced into the drainback tank, considerably less lift is required for circulation. The pressure drop requirement is lowered and system circulation becomes comparable to a closed loop system. Once water is flowing throughout the loop, the second pump is “time delay to off” until the next cycle of solar collection begins and total lift is required once again. This option allows for the use of smaller, less expensive pumps and increases electrical efficiency.
A variety of options are available for the mechanical transfer of energy from the drainback reservoir to the solar storage tank. Drainback thermal transfer may include a heat exchanger in the drainback tank, a heat exchanger in a hot water storage vessel or an external heat exchanger.
Drainback reservoirs are sized large enough to displace the volume of air in the collector loop with water while maintaining a flooded drainback pump. If there is a coil heat exchanger in the drainback reservoir, it must be completely submerged during the collection cycle. Normally a site glass on a drainback reservoir is used as a gauge for fill. Drainback reservoirs are manufactured in 10-, 15-, 20-, 40- and 60-gallon sizes, with stainless steel tanks.
Reservoirs for larger arrays may be piped together in parallel to accommodate volumes over 60 gallons or when port capacity requires more flow (port size may be limited to ¾ or 1 inch). It is standard practice to install the drainback reservoir on a wall stand above the solar storage tank. This helps position the drainback pump below the bottom of the reservoir. The pump should be installed vertically and should pump against gravity to avoid pump cavitation. For better pumping characteristics, a drainback loop can be sealed closed so that it is airtight and the entire loop charged to 10 psi or more. Pressurizing a drainback closed loop raises the boiling point while compressing entrained air out of the working fluid during circulation.
For larger systems with an external heat exchanger, two pumps may be incorporated on the drainback reservoir; one for the solar collector loop and the second for the reservoir heat exchanger loop. This design separates two (usually high) pressure drops into two loops. Separating the loops helps size for smaller pumps and enables a more efficient control sequence for heat transfer, since the drainback tank can now be first “primed” with heat, prior to transfer to the solar storage tank.
External heat exchangers allow for additional flexibility with the installed location of the solar storage tank. Since pressurized water is circulated from the heat exchanger to the solar storage tank, the solar tank can be installed in any location relative to the drainback tank. Limitations may occur for long pipe runs and heat loss but, if necessary, a solar storage tank may be installed above the collector height.
Many early large drainback systems had insulated fiberglass-reinforced plastic tanks, which are still quite popular today. Water from these tanks flows directly into the collectors. These tanks contain stainless steel or copper heat exchange coils sized in length to accommodate a large enough surface area for a sole pass high temperature rise. As an example, cold water is preheated through the submerged coils in the atmospheric tank, raising the cold inlet temperature close to the solar tank temperature en route to the backup water heater.
Active solar hot water heating designed properly is an efficient method to collect, store and distribute solar thermal energy. As the solar thermal industry matures, drainback will most likely gain higher ground and increased market share for commercial systems due to the most basic demand of building owners and their maintenance staff; desire for a high performance system with a long life cycle, designed for low maintenance and priced for value.
Owners may be uncomfortable with a closed loop system if the building’s hot water load varies or shuts down from time to time, such as in a school. Those manufacturers, designers and installers who don’t get involved with drainback will do just fine with closed loop system design and installation. But those who have a complete knowledge of both system types have an improved opportunity to satisfy their customers’ requests for the simplicity drainback systems provide.
Both closed loop and drainback systems will satisfy performance expectations when designed and installed correctly. The question is which system is better suited for a specific commercial application and hot water load variations. Drainback could, in fact, provide simple solutions to some of the complex issues closed loop systems present.