Mother's Do It Yourself Solar Water Heating Project
(Page 2 of 4)
September/October 1981
By Mother Earth News Editors
Our results reflect the average number of British Thermal Units (one BTU is the quantity of heat required to raise one pound of water 1°F) produced per square foot of collector, per hour, over the course of the period extending from the beginning of the morning heating cycle until the collectors reached maximum heat level (usually about 140 °F, by 1:00 p.m.).
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Finally, as is the case with any test, our numbers are only as accurate as were the monitoring equipment and methods used. By calibrating our Heliotrope General SAS-3 thermistors before and after the evaluation (and disqualifying any of the "thermometers" that had more than 3% inherent variation from the mean), taking flow measurements every half-hour with a graduated cylinder and stopwatch, and using a quality digital ohmmeter to measure thermistor resistance, we feel that we have achieved an overall accuracy of plus or minus 10%. That means that the numbers we provide could be off by 10%, in either direction, from the correct figure.
FIRST, A CHANGE IN THE WATER HEATING TEST
During the evaluation, the already assembled commercial collector (which we had hoped would provide a control to which we could compare the kits and homebuilt units) performed quite poorly. We opened it up in an effort to locate the problem, and discovered that the panel obtained from a local retail outlet—was not what it had been billed to be. It wasn't manufactured by American Solar King, didn't have a black chrome absorber ...and, in fact, had no identification on it at all. Although the unit was less than a year old, paint was already peeling from the copper plates. Such a device obviously wouldn't give an adequate reference for comparison purposes, so we've entirely eliminated it from the final results. (To be sure you get what you're after when shopping for a collector yourself, look for an identification badge pop-riveted to the frame before making a purchase ...there do seem to be some poorly made "counterfeits" on the market.)
SOLAR ENERGY
When our projected test accuracy of t 10% is taken into consideration, the BTU Bucket, the Suntree, and MOTHER's double-plate homemade in-line collectors don't have significantly different outputs. Again, our evaluation's potential error indicates that the SolaRoll panel yielded roughly 8% fewer BTU than did the average of the three top-rated units. MOTHER's single-plate in-line lagged 20% behind that same average.
