The average efficiency for the heating system over this day was 48%, and that’s not too shabby, especially if you consider that these collectors are refurbished. They only use flat-black paint on the absorber, and are put together without any welds between the riser tubes and absorber plate. (Having a selective coating on the absorber and a solid bond between the riser tubes and absorber plate would be even more efficient.)
Let’s look at the same data for two warmer days during the year. The February 29 graphs show data collected on a day when the average ambient temperature was about 62°F. Again, the irradiance is about the same, but a higher efficiency is evident. This is due mainly to the much lower Ti – Ta. value. More energy is the result.
For the March 3 graphs, the average ambient temperature was 43°F—almost 20°F higher than the first scenario. However, the average irradiance, DT, and energy output are almost the same. How can this be? Remember that the shop temperature is allowed to float—there are no thermostatic setpoints, so the interior temperature increases as the ambient temperature increases—and notice it is much warmer this time around, as evidenced by the higher supply temperature. The energy output is probably higher due to the lower average Ti – Ta. Basically, the system puts in about the same amount of energy whether it’s warm or cold outside because the interior temperatures will adjust to maintain a certain level above ambient.
I was surprised to see how consistent the efficiency and energy output were over the heating season, since the general expectation would be to see higher output with warmer ambient temperatures. I doubt this to be a general result, and it is mostly due to a seasonally variable building interior temperature.
Now that you’ve seen how my system performs, let’s ask a critical question: “Is it worth it to put solar thermal space heating in a building?” I’ve been a fan of these systems my entire career and I wish I could answer with a resounding, “Yes!” However, from experience with design and performance in residential applications, the answer is a more qualified, “Maybe.”
Here’s a consideration: Would an active solar hydronic system add more heat energy to the building than an equivalent passive system of vertical south-facing windows? In my case, during the coldest period of the year, such glazing would put in about 80% of the active system’s heat on a sunny day—and that includes the 24-hour losses that windows experience. During warmer times of the heating season, the contribution from passive solar heating would be even higher, because of lower heat losses through the windows.
My calculations included the insulation levels in my building envelope and double-paned windows with U-values of 0.5 and solar heat-gain coefficients of 0.6. There are some other considerations, however. On cloudy days, the windows lose energy 24 hours per day, while the active system just shuts down. If you have a lot of cold, cloudy days during the heating season, an active system starts to look more appealing.
Passive solar windows will be net losers in cloudy weather, and the active system will only contribute energy when there’s enough to be collected. Active systems provide heat without nighttime and inclement weather losses. In my case, the passive solar window area is just about equal to the area of the active collectors (about 80 square feet for each). In my climate, the contributions of each are going to be roughly equal, with an advantage going to the active system. This combination of passive and active heating allows me to have a combined collection area equal to 26% of the building’s floor area. This is a high percentage, but is acceptable because I have sufficient thermal mass to store the incoming energy during sunny days. It would be difficult to deal with all of that area if I had only passive windows, because of excessive light and structural issues.
If I had to build the shop all over again, I would have included more envelope insulation, which would slow heat escape during the winter and let in less heat during the summer. There’s plenty of thermal mass in the concrete floor and concrete-filled block walls, but that’s no substitute for insulation, even in the sunny Southwest. I’d also be more careful to control losses from air leakage—there’s a large insulated garage door on the building that doesn’t seal well. Finally, I’d buy SRCC-rated collectors, which would offer better performance and higher output mostly due to the absorber construction and surface coating.
Carl Bickford is a professor of engineering and renewable energy at San Juan College in Farmington, New Mexico.
SRCC • www.solar-rating.org
Data Logging Equipment Manufacturers:
Analog Devices Inc. • www.analog.com • AMP 04 instrumentation amplifier chip
Apogee Instruments Inc. • www.apogeeinstruments.com • Pyranometer
Li-Cor Inc. • www.licor.com • Pyranometer
Onset Computer Corp. • www.onsetcomp.com • Hobo data loggers & sensors
“Principles of Radiation Measurement” • www.licor.com/env/PDF_Files/Rad_Meas.pdf