Having issues with solar water heating (SWH) collector performance? A thermal imaging camera can simplify the diagnosis.
Last year, a homeowner with a five-year-old SWH system contacted me for a second opinion. She told me that the designer claimed she was expecting too much, but she was sure that the system had produced more hot water when it was first installed.
Upon arrival, I performed an initial system inspection. The fluid pressure was within the normal operating range for an antifreeze-based, closed-loop system. The differential controller settings were reasonable, and the flow meter indicated that the pump was circulating the heat-transfer fluid (glycol) at an appropriate flow rate. After verifying that the temperature sensor in the storage tank was properly installed and operating, I headed outside to inspect the collectors on the roof.
Diminished collector performance can result from a variety of factors. In systems in which potable water is heated directly by the collectors, calcification in the collector or pipes may occur due to high-mineral-content water. Deterioration of the absorber surface may occur depending upon the quality of the absorber coating and the age of the collector.
Most flat-plate collectors tend to be quite durable but will show deterioration toward the end of their service life. These issues may include leaks where the riser tubes are joined to the headers or failure of the bond between the riser tubing and the absorber plate. There may also be issues related to failure of the glazing seal.
Evacuated-tube collectors are inherently less durable. This is primarily due to the type of glass used (nontempered borosilicate or soda-lime glass) and the need for the collectors to maintain a vacuum to ensure proper performance. A crack or leak around the seal of an evacuated tube will lead to the loss of vacuum, rendering the tube ineffective. When evacuated tubes are manufactured, a barium getter is fired inside the vacuum. If there is oxygen remaining in the tube after the vacuum has been created or there is a small leak, the getter will serve to maintain the vacuum by consuming this oxygen. The getter is typically made of barium, which is silver. As the getter absorbs oxygen, it becomes barium oxide and turns white. The silvery surface is an indicator of the integrity of the vacuum.
While the barium getter can provide visual confirmation of whether or not the vacuum is intact, it is not always simple to observe whether the getter is still present. For instance, some tube manufacturers apply the barium on the tubing behind the absorber. With collectors mounted on tilted racks, the getter can be observed from behind. But the getter on parallel-mounted collectors, which lie close to the roof, may be difficult to observe. Other manufacturers apply the barium getter to the lower end of the tube.
Infrared cameras are particularly effective at diagnosing issues with evacuated-tube collectors without needing to go on a roof. Where standard cameras capture visible light, IR cameras capture infrared radiation and indicate the heat with color or shades of gray. A properly configured IR camera can indicate the surface temperatures of various objects, including solar collectors.
Although not as prevalent, issues with flat-plate collectors can be diagnosed with IR camera technology. If there is a significant variation in flow between two banks of collectors, the bank with the higher flow will have a cooler surface since the heat is being removed faster compared to the bank with the slower flow rate. In large banks of flat-plate collectors, the flow through the collectors may be uneven. An IR camera can diagnose whether the center of the bank is hotter than the outer edges as a result of uneven flow rates. In older flat-plate collectors, an IR camera may be able to assist with diagnosing the detachment of the absorber from the riser tubing or failure of the glazing seal by illustrating temperature variations on the collector glazing or indicating higher temperatures around the seals.