IR cameras are especially helpful in diagnosing problems with evacuated-tube systems, since evacuated tubes that have lost their vacuum will have a warmer surface temperature than ones that are intact. This is due to the loss of insulation that occurs when the vacuum has failed. If the getter is difficult to observe due to the boots on the tubes or its location, or if accessing the roof is a challenge, an infrared camera can save time and avoid the risks of climbing on the roof.
An infrared camera can also be used to diagnose issues that are not visible. For example, some brands of evacuated-tube collectors have showed manufacturing or durability issues with their heat pipes—thin copper tubes that contain a small amount of liquid within a vacuum. The vacuum allows the fluid to boil at roughly 85°F. The vapor rises to the condenser bulb, where it transfers heat to the cooler heat-transfer fluid in the manifold. This condenses the vapor into liquid, which sinks back to the bottom of the heat pipe. But if the heat pipe loses its vacuum, the fluid won’t boil until it reaches a much higher temperature, causing the heat-pipe efficiency to drop dramatically.
As a result, since much higher temperatures are required to vaporize the liquid, the tubing in a failed heat pipe is hotter than one that is working properly. However, the condenser bulbs are cooler, because the vaporization and condensation cycle is not occuring. This additional heat is revealed by the IR camera when compared to one that is working correctly. This difference is also detectable at the condenser bulb, which is unable to reach standard operating temperature since the fluid in the heat pipe is often unable to boil and accumulate the same level of heat in the condenser bulb.
When I received my client’s initial call, I was well aware that the model of evacuated tubes in her system had a history of heat-pipe failures. Since the roof was easily accessible, I removed a random selection of tubes from the array to compare their performance to a new tube. Before I placed the tubes in the sun, I wrapped the condenser bulb of each heat pipe with black stretch-and-seal silicone tape, which won’t leave a residue on the condenser bulb.
The infrared radiation measured by the camera is a combination of the IR radiation emitted by, transmitted through, and reflected by a surface. Since the copper heat pipe is smooth and shiny, it reflects a significant amount of IR radiation. Without the silicone tape, it would have been difficult to measure the actual condenser bulb temperature due to the reflected infrared radiation from the cold sky. Flat, black surfaces are less reflective, hence the black silicone tape.
Upon an initial visual inspection, I spotted a few heat pipes that had a small rupture at the bottom of the heat pipe, likely due to a freeze failure. Outside of the minority of the heat pipes that exhibited this issue, most of the heat pipes appeared fine. However, the IR camera indicated otherwise. After allowing the tubes to sit in the sun for 15 minutes, I measured their temperatures with the IR camera. The new heat pipe had a condenser bulb temperature of nearly 300°F. The condenser bulbs from the three evacuated tubes I had removed at random from the array had temperatures ranging from 115°F to 120°F. The huge difference in temperature illustrates the importance of the vacuum in the heat pipe. Without the vacuum, the fluid in the heat pipe is unable to boil and cannot provide significant heat to the condenser bulb.
A few months later, I received a call from another homeowner about a poorly performing system—a single 30-tube collector mounted parallel to a standing-seam metal roof. I was unable to observe the barium getter in each tube since it was hidden by the absorbers, and access to the array was precarious.
From a ladder, I was able to take IR photos of the tubes. I manually adjusted the camera’s span and level (see “Camera Settings” sidebar). I also reduced the emissivity setting for the surface to 0.60 so I could compare individual tubes. While this setting does not provide absolute temperature readings of the glass surface, it was sufficient for comparing the relative temperature of the vacuum tubes. The IR camera allowed me to pinpoint the failed tubes without having to get on the roof.
The photos were enough to file a warranty claim, and I didn’t have to access the roof until I had the replacement tubes. Once I was able to observe the getter, I discovered that all but one of the failed tubes had lost their vacuum. The other tube was likely the victim of a stuck snap-switch in the heat pipe that isolates the condenser bulb from the rest of the heat pipe when temperatures exceed the preset maximum.
Without an IR camera, I would have had to install fall protection, access the roof, and rotate individual tubes to look for indications of a failed vacuum. This approach would have only identified the eight tubes that lost their vacuum; I likely would have missed the tube with the heat-pipe (snap-switch) issue.
Applications are not limited to collector arrays. An IR camera can also help identify issues like failed mixing valves or excessive heat loss in a system.
Vaughan Woodruff owns Insource Renewables, a Maine-based renewable energy company focused on the design and construction of solar energy systems. He also provides training for Solar Energy International and is working with the New York State Energy Research and Development Authority to train code officials on solar heating technology.