Variations On Residential Solar Water Heating PART 2
In the spring issue of the Solar Heating Report we discussed the current “mainstay” method of freeze protection for residential solar water heating systems in North America. That method relies on an antifreeze solution as the working fluid in a closed hydronic circuit between the collector array and a heat exchanger for the storage tank.
This month we’ll look at two other approaches that eliminate the need for antifreeze solutions - draindown systems and drainback systems.
It’s probably obvious from the “drain” portion of these classification names that both approaches count on getting water out of the solar collectors and any exposed piping when freezing conditions are present or imminent. However, that’s where any similarity ends, so be sure to distinguish carefully between the “down” vs. “back” portions of these names.
Draindown SystemsA draindown solar water heating system is an interesting concept, but in practice is likely to prove that Murphy’s Law is alive and well. The concept is shown in Figure 1.
In draindown systems, domestic water passes directly through the collectors. Although this sounds great from the standpoint of heat transfer, it doesn’t bode well for the copper absorber plates in locations with hard or otherwise aggressive water.
Beyond this is the sequence of operations necessary to protect the system from freezing. Here is how the system is supposed to work:
A third temperature sensor near the bottom of the collectors signals the system controller when it detects a near-freezing condition (perhaps a temperature around 35 degrees F). Power is then removed from the two normally closed solenoid valves in the supply and return piping to the collector array. This isolates the storage tank from the portion of the system subject to freezing.
Power is also removed from a normally open solenoid valve, which allows water in the collectors and exposed piping to drain out of the system. Air enters the system through the vacuum breaker at the top of the collector array to expedite drainage.
When the freezing condition is no longer present, the system refills with fresh water. Air is pushed out through a vent at the top of the collector array, and the system awaits the next solar collection opportunity.
When freezing is not detected, the system doesn’t drain. At the end of the collection period, the circulator stops and the normally closed solenoid valves go to their closed position to prevent thermosyphoning.
Sounds pretty straight forward, doesn’t it? But imagine what happens if the freeze detection sensor drifts slightly out of calibration, or one of the solenoid valves fails to operate properly, or some other “fluke” occurs to the expected sequence of operation. The inevitable result is a hard freeze of the collectors and a very expensive repair. Draindown systems had their shot at the solar water heating market more than 30 years ago. I was there as a witness, as were others. Ask any solar practitioner of this vintage their opinion on draindown systems and you’re likely to get the same advice - don’t do it. I concur with that response.
Drainback SystemsWhat a difference a word can make. In this case it’s the word “back” rather than “down.” A drainback solar water heating system is the essence of simplicity. It relies on a drainage mechanism that never fails - gravity. Simply put, whenever the collector circulator is not operating, water in the collectors and any exposed piping runs back to a holding tank within heated space. No antifreeze, no check valves and no special freeze detection controls are needed.
What is needed are properly pitched collectors and piping. A minimum pitch of 1/4 inch per foot is required to ensure complete drainage of all piping and hardware subject to freezing temperatures. If you can’t provide this pitch, don’t install a drainback system.
When it comes to pitching the collectors, there are a couple of options as shown in Figures 2a and 2b. In smaller systems with one to four collectors, it may be easiest to pitch the entire array, and then pipe it for reverse return flow as shown in Figure 2a. The latter helps ensure equal flow distribution through each collector.
If the array will be larger, it often makes sense to divide it in half, and create two opposing slopes toward a common low point, as shown in Figure 2b. With either of these arrangements, be sure to verify any specific detail required by the collector manufacturer for the “downslope dead end” of the collector header piping, where a small amount of water may reside even after the remainder of the collector has emptied.
In this system, the space at the top of the solar storage tank serves as the drainback reservoir as well as the expansion chamber for the collector circuit. The water level in the solar subsystem is set using the sight glass on the collector supply pipe. Every time the circulator turns off, water returns to this level - usually in less than a minute. It remains at this level until the next solar collection period begins. At that time the circulator pushes water back up through the collectors. Eventually this water flow pushes all the air back down the return pipe and into the solar storage tank.
The return piping should be sized for a flow velocity of at least 2 feet per second to ensure all air is returned to the tank, and that a siphon is established over the top of the collector circuit.
Pressurized domestic water passes through an internal copper or stainless-steel heat exchanger suspended in the solar storage tank. Depending on the tank’s temperature, one pass through this coil may provide all the heating required. If not, this system passes the solar preheated water to a conventional water heater for a boost. A modulating tankless heater could also be used for auxiliary heating.
The solar storage tank in a drainback system may be pressurized or unpressurized. The latter type of tank is usually less expensive, but there will be evaporation of water over time, and the owner needs to maintain the water level. Also, because an unpressurized tank is vented to the atmosphere, the collector circulator must be either bronze or stainless steel. If the tank is pressurized, water loss should not occur and a cast-iron circulator is fine.
Figure 4 shows another variation for a drainback system. A separate tank serves as the drainback reservoir. It has been elevated above the main storage tank to minimize the lift head required of the collector circulator. This detail makes it well-suited for multistory buildings. Solar-heated water passes through an internal heat exchanger in the solar storage just like in an antifreeze-based system.
More To FollowWriting an article on solar water heating is like writing an article on radiant panel heating; there’s always more to tell - more details on design, piping, control, sizing and advantages/limitation of different approaches. We’ll explore these in more depth in future issues of the Solar Heating Report.
In the meantime, I recommend that readers who want to get started with solar water heating explore the “turn-key” systems now available in North America. These pre-engineered packages include just about everything you need for a successful installation, and are much simpler and faster to install than those I started with 30 years ago. You can find a list of SRCC/OG-300-approved solar water heating systems under the “Ratings” tab at www.solar-rating.org.