“expecting.” So, when looking at a selective surface, the common IR thermometer will display a false low temper- ature, because it calculates the temperature based on a higher typical surface emissivity value. Once you know this, the information can be used to your advantage. The false low temperature of one surface can be compared to another to give a relative comparison of the emissivity that the IR scanner “sees.” Under the same temperature
The high absorptivity of the black plates has been improved as well, which is clearly visible when viewing the reassembled collectors that now have an obvious dark- er and cleaner appearance. The overall absorptivity values have been estimated as follows, based on the change in color of panels before and after cleaning.
Figure 35-6 Figure 35-5
conditions, comparatively lower IR readings indicate lower emissivity. Figure 35-5 shows a group of readings taken with an IR
scanner calibrated to 0.95. The frame coated with black paint reads accurately at 141 F, which is the actual tem- perature of all the surfaces at that moment. The cleanest selective surface reads near zero, which indicates that it is doing its job as a low emissivity coating, holding on to the thermal radiation. The selective surfaces that are coated with dust and minerals read higher temperatures, showing that they need to be cleaned and restored to take advantage of that Low E benefit.
Summary Figure 35-6 shows one of the solar collector banks after cleaning. Based on the observations, measurements and estimations, the results of this solar collector refurbish- ment effort can be summarized as follows for the overall surfaces of all the collectors combined. The low emissivity of the black plates was substantial-
ly improved by the cleaning process. The following values are estimates based on comparison and interpolation of the IR temperature readings. Before cleaning – 0.51 After cleaning – 0.28 Original value – 0.17 This means that the solar heat loss by thermal radiation
has been reduced from a rate that was three times higher than the original to a rate that is only 1.6 times higher on average. Another way to say this is that the solar heat loss by radiation has been nearly cut in half.
Plumbing Engineer
Before cleaning – 0.71 After cleaning – 0.90 Original value – 0.94
Final notes These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. n
Bristol Stickney has been designing, manufacturing, repair-
ing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating pro- fessionals. Visit
www.solarlogicllc.com for more information.
In this series of articles, I have been making the case that
the key ingredients for solar/hydronic design and installation can be divided into six categories, roughly in order of their importance.
1. Reliability 2. Effectiveness 3. Compatibility 4. Elegance 5. Serviceability 6. Efficiency The success of any solar hydronic home heating installa-
tion depends on the often-conflicting balance between any of these six principles. Finding the balance between them defines the art of solar heating design.
The views and opinions expressed in this column are those of the author and do not reflect those of Plumbing Engineer nor its publisher, TMB Publishing.
June 2011/Page 31
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