I’m curious about flow rate through solar water heating collectors and how that affects an SWH system’s efficiency. On one hand, it seems like a slower rate would allow more heat gain, since the water spends more time in the collector. But I also understand that efficiency is higher with water that is at a lower temperature. So then I assume that more water moving through the collector more quickly (even if it’s not getting as hot per cycle), is actually more efficient overall. Either way, how is a pump sized and flow rate optimized for maximum efficiency?
Ralph Warren • Charlotte, North Carolina
A good way to understand the flow rate is to evaluate it conceptually. Hot collectors don’t do you any good, but a tank of heated water does. Since collectors are outside, they lose some of their collected heat whenever the collector is warmer than the surrounding air. This is the normal situation when trying to heat a tank of water to 120°F or more. The greater the difference in temperature between the collector and outside air, the greater the heat loss.
The flow rate through SWH collectors should be high enough to keep the difference in temperature (delta-T) between the collector’s inlet and outlet to about 30°F or less. This low delta-T is achieved with a higher flow rate. A system that has a high delta-T, perhaps 50°F to 150°F, has too low of a flow rate—the result is a hot collector. This difference in the delta-Ts can mean as much as a 50% loss over the day in the hot collector, all due to the extra heat lost to the surrounding air. The lower delta-T, higher-flow-rate collector puts more hot water in the tank, since less heat is lost.
A high delta-T can have numerous causes: a pump or piping that’s too small; a heat exchanger without enough capacity; or a pipe obstruction. A low-head pump (less than 10 feet) is sufficient for systems with one or two collectors. A medium-head pump (10 to 20 feet) is appropriate for systems up to six collectors. A high-head pump (more than 20 feet) is normally required for drainback systems and those with more than six collectors. These pump recommendations assume 3/4-inch tubing for small systems and 1-inch tubing for larger systems.
Chuck Marken • Home Power solar thermal editor
For more on collector efficiencies, see “Get into Hot Water” in HP123.
For more on pump sizing, see “Pick the Right Pump” in HP121.