At the end of last month's column Hydronic radiant heating options, Part 3, I promised to tell you more about radiant glass windows. As I stated in the last column, this device seems to defy normal logic in more ways than one. It not only had ice forming on a surface that was ½-in. away from a significantly warmer surface, but it also acted like a thermal capacitor. The energy input to the window is being suppressed due to the operating limitation placed upon it by the UL approval of 115°F. In other words, the window is operating on pulsed power, or pulse frequency modulation. This is similar to pulse width modulation except that it is possible to vary the voltage to the unit as opposed to keeping the voltage at a constant and varying the device on and off time.
“What's the hottest temperature you've seen from this technology?” I asked one of the inventors. “Oh, we got it up to around 350°F one time when we poured straight 120V DC power to it. Of course we can't do that under our listing unless it's in a controlled laboratory condition, which it was,” he responded.
That is obviously way too hot for typical window application. But my mind was in other places. I was thinking about how we could take this technology and apply it to off-peak energy storage situations, or use it to store excess solar PV power or excess electrical production from the electric side of a hydrogen fuel cell generator.
“When does the buss bar have issues with operating temperatures?” I asked. “Oh, it's well in excess of 500°F,” one of the inventors told me.
That's amazing, simply amazing! Here is a device that could eliminate the normal heat loss through a given window and, if necessary, be programmed to produce radiant heating from the window to flow into the room. What more could a comfort contractor ask for?
“How much heat does it put out?” I asked. “Well, we're not exactly sure about that question. We are glass manufacturers and electricians. We know about those things, but not about heating things. That's why we asked you to come take a look at it.”
They had done a significant amount of research on their own. They had recorded by hand the different operating temperatures of the freezer, the room in which the freezer was located (their R&D lab, if you will), the temperatures of all four glass surfaces, and they had done a significant amount of viewing through the lens of a FLIR infrared camera. They had hired a mechanical engineer with significant experience in the field of windows, and they had a fairly good idea of how well it was working to affect human comfort. However, they still were unsure as to how much energy these wonderful units would put out under a given set of circumstances.
Enter Kansas State University's National Gas Machinery Laboratory.
This prestigious facility, under the direction of well-known radiant guru Dr. Kirby Chapman, took on the challenge of testing the thermal performance of the radiant glass window. Bear in mind that this is relatively virgin territory. To the best of my knowledge, there is only one other architectural radiant glass window product available, and from what I can tell, it has never been tested for thermal output capacity. It is promoted on its Website for its extreme increase in human comfort, but there is no mention of its thermal output capacity or its net thermal efficiency (inward flow versus outward flow of energy from the heated glass).
So it was off to KSU for two of the sample-sized radiant heated windows and two of the co-inventors. They hand delivered the windows to the world-class testing facilities at KSU. They met Dr. Chapman, along with the students and assistants who operate the nationally recognized testing facility, and the inventors assisted in the installation of the windows into the “test” module. The test module was essentially a plywood cube (approximately ½-in. thick). It was 3½ feet wide, 3½ feet deep and 4 feet tall. This is what they called their “controlled environment.” The radiant windows were placed in holes cut into one wall of this controlled environment.
There was another small electric heater placed within the controlled environment. There also were numerous temperature sensors placed within the space into a “comfort ball” to detect mean radiant temperatures, along with 10 flux sensors on both sides of the glass (five per side). The whole apparatus was then placed inside of the NGML environmental test chamber, which is a rather large, extremely precision controlled environmental freezer.
The windows were subjected to steady state operating conditions at three different “outdoor” exposure conditions. These conditions were simulating outside temperatures of 10°F, 20°F and 30°F. All tests were performed twice to guarantee data reliability and data integrity. That is the sign of a truly “controlled” scientific test.
Tune in next month as we discover the actual thermal performance of this newest exciting technology on the war against human discomfort and energy waste.
Until then, keep your nose pressed against the glass and away from the grindstone.
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.