Used in tropical settings where freezing never occurs, this is the simplest of the active systems, using a pump and a standard tank with electrical elements teamed with a solar thermal collector. A direct-pump system is also known as a “direct-forced circulation” system by SRCC classifications. In this open-loop system, the collector-loop fluid is potable water. As with ICS and open-loop thermosyphon systems, potable water must run outside to the collector, and the associated plumbing is vulnerable to freezing. A weaker freeze link is the smaller riser tubes connected to the header tubes. They are subject to freezing before the insulated potable water lines. Direct-pump systems can easily be married to a PV module that will power a DC pump. Direct forced-circulation systems are very popular in places like Hawaii, which has mild temperatures and plenty of sunshine.
These indirect forced-circulation systems are reliable, freeze tolerant, and fairly easy to install. The closed-loop drainback system requires perhaps the least amount of maintenance of any indirect, active system. The heat-transfer fluid is distilled water, which seldom has to be changed. When the system is not pumping, the solar collector is empty with the water having drained to the reservoir tank, usually located just above the solar storage tank. Higher-capacity reservoir tanks are typically required in large systems. The system relies on the collectors and piping being drained when freezing conditions are possible. Both are sloped toward the drainback tank so that when the pump turns off, the water in the collector loop passively drains back into the tank.
Since the pump must have enough power to push water from the drainback-tank fluid level to the top of the collectors—a distance that can be 20 or 30 feet—most installations require a high-head pump. Because of the head requirements and the limited choices in DC pumps, drainback systems are tougher to adapt to direct PV power. (Drainback systems are featured in HP86 & HP97.)
This is the most complex indirect forced-circulation, closed-loop system—and therefore the most difficult to install. It also is the system with the best freeze protection, and as such, is popular in northern climates.
In this active, closed-loop system, incoming potable water is routed to the solar storage tank, but never into the collectors. A water–antifreeze mixture circulates from the collectors through a heat exchanger and then is pumped back through the collectors.
Antifreeze systems can overheat in the summer if there is too much collector surface area relative to tank storage volume. Overheating can be combated by using the “vacation mode” of newer differential controls, which will allow fluid to circulate through the system at night, cooling the fluid. PV-powered systems can incorporate a bypass valve around the check valve, which will allow the system to reverse thermosyphon at night to cool the antifreeze. (See Bob Inouye’s article in HP123 for details of a bypass valve. Antifreeze systems were covered in depth in HP85 & HP95.) For a good overview of the five systems mentioned here, see “Solar Hot Water: Simplified” in HP107.
Once you’ve classified your climate, you can determine what system is right for your site. What’s best? If you live in a freezing climate—or in a milder climate but just want to hedge your bets, use a drainback or antifreeze closed-loop system. If it doesn’t freeze, or freezes are rare and mild, one of three “mild-climate” systems can fit your needs. Passive or active, PV or AC powered, these choices are up to you. Systems with quality components should have decades of good performance.
Contributing editor Chuck Marken is a New Mexico-licensed plumber, electrician, and heating and air conditioning contractor. He has been installing and servicing solar thermal systems since 1979. Chuck is a part-time instructor for Solar Energy International and the University of New Mexico.
Solar Rating & Certification Corp. • www.solar-rating.org