Proper sizing is the first step toward an effective SHW system (see “Sizing Solar Hot Water Systems” in HP118 for details). If not properly sized—either in terms of demand or in the ratio of collector to storage space—the system can be ineffective or overheat. Collector angle is also important. It is common to see collectors flush-mounted on roofs with a 3:12 to 5:12 pitch (less than a 25° tilt). In most locations, this will result in increased summer production, when the sun is high in the sky, and reduced winter production, when the sun is at a lower angle. It also increases the likelihood of summer overheating. A steeper tilt angle will improve winter performance and help mitigate summertime overheating.
Systems that are designed for meeting winter loads are typically large, using multiple collectors to produce enough heat during shorter days with less solar energy. When there is less demand for hot water, such as in the summer, the systems can overheat. Drainback systems are often preferable when overheating is a factor.
Some systems use temporary covers on some of the collectors to reduce overheating. For space heating or snow-melting systems, another option is to drain the HTF from the system during the time of year when it is not needed. These strategies require effort twice a year to cover/uncover the collectors or drain/recharge the loop. The added cost and time, along with the potential for forgetting to do it, make these strategies problematic.
Another option is to incorporate some sort of heat dump like piping in the ground to transfer heat to the earth, preventing the system from stagnating. Or a second, off-season load can be added—a heating or snow-melt system may double as a pool-heating system in the summer. Note that more complex controls and plumbing will be required for these strategies; the more automated they are, the more effective they’ll be.
Another overheating potential, especially for residential SHW systems, occurs during periods when the system isn’t being used. Unoccupied homes (such as during vacation) with the corresponding drop in hot water use can easily lead to overheating, as the system hits higher and higher temperatures each day.
One strategy is a vacation bypass valve—a loop in the plumbing around the check valve. Normally, the ball valve in this bypass loop is closed, with flow through the check valve. Opening the ball valve allows a thermosyphon loop to bypass the check valve, and can result in the tank cooling, helping the system to avoid stagnation. But this strategy relies on the user, since bypassing the check valve requires manually opening the valve, and then reclosing it to resume normal operation. If accidentally left open, system performance can suffer dramatically.
This method also requires that a thermosyphon loop can occur, which is not always the case—it depends on the pipe run’s length, height, and diameter. Thermosyphoning also does not work with evacuated tube or heat pipe type collectors, because the vacuum prevents night radiation of the heat. If an external heat exchanger is used, additional plumbing may be required to ensure that both sides—the HTF loop and the end-use water—are thermosyphoning.
A better strategy, and one available on several differential controllers, is “vacation mode,” which runs the pump at night if the storage water exceeds a preset temperature. This cools the system and prevents stagnation. Depending on the controller, the vacation setting may be able to be left on. Use caution if this is the case, as an improper (too low) setting can lead to poor system performance. Other controllers have to be put into vacation mode manually, again putting the responsibility on the end user. As is the case with a bypass valve, this strategy will not work with evacuated-tube collectors.
Brian Mehalic is a NABCEP-certified PV installer who has experience designing, installing, and servicing PV, solar thermal, wind, and water-pumping systems. He is an instructor and PV curricula developer for Solar Energy International.