Article by Steve Baer
Zomeworks Corporation
NMSEA SunPaper — March/April 2003
For years I have tested solar collectors by using them to heat water for my shower. (Fig. 1: Energie Solaire's selective surface collector on top of retired glazed collector; plumbed with ¾" un-insulated hose.)
I have a 30-gallon plastic drum set above the shower with a float valve and gravity feed to the showerhead. Lines drop outside to the collector, which thermosiphons to the tank. This year I have tried something I should have tried thirty years ago: leaving the glazing off the collector. I couldn't have been more surprised to find that during icy clear January weather (high 46° F), and without glazing, two of our 32" × 48" blow molded, polypropylene Skymat™ radiator absorbers (10 square feet each) gave me a warm shower the next morning. Not hot, as I didn't need to mix with cold. (Fig. 2: Three 32" × 48" Skymats plumbed to 50 gallon low pressure tank.)
I used these absorbers for weeks of warm showers until a defective Skymat™ ruptured. Then I switched to something even more astonishing, a single, unglazed 10 square foot collector made by a Swiss company, Energie Solaire. I had already seen this absorber, made of selectively coated stainless steel, stagnate above boiling with no glazing on a cold, partly cloudy, breezy November day. The selective surface absorbs visible light (it looks black) but doesn't radiate infrared. (White paint is a selective surface in reverse. It reflects visible light and radiates infrared.) Sure enough, I have continued to get warm (sometimes even hot) showers during these last weeks.
Energie Solaire doesn't advertise their absorbers as unglazed for water heating, but after my experience these last months I am more convinced that solar water heaters can save money.
Once the glass is off the collector, the absorber can't set fires. If the house has flat composition shingles we can lay the bare absorber directly on the roof, which serves as back insulation. We don't need a collector box with sides, insulation and a back-plate. An unglazed absorber is much less expensive than a boxed collector. If the roof angles are not ideal, we can use a larger absorber.
Thirty years ago at Zomeworks we built a number of solar collectors that were spirals of black polyethylene pipe housed in glazed boxes. We pumped water through them with Bell and Gossett circulating pumps. These were freeze tolerant (I don't remember for how many winters) but not tolerant of the high temperatures that occurred in the boxes if the pump failed. The plastic bloated like a snake that's eaten a rabbit. I recall that we found high molecular weight polyethylene more rugged than regular polyethylene. I wonder today if we'd have been more successful had we removed the glazing from the collectors.
The stagnation temperature drops when glazing is removed. However, don't forget what the pool heater people know so well: unglazed collectors are more efficient than glazed, at temperatures close to ambient. While the glazing stops airflow past the collector and blocks radiation to the cold sky, it also reduces sun reaching the absorber.
Which to bet on; glazed or unglazed? Bright sun, still weather and a selective surface raises the temperature. Energie Solaire posts excellent information on their web site www.energie-solaire.com. This includes performance charts for the absorber I have on my water heater, both glazed and unglazed. In New Mexico's strong sun — 1,000 W/m² — the charts show unglazed collectors to be more efficient all the way to a temperature 36° F above ambient. If the afternoon high temperature is 50° F the selective surfaced unglazed collector works better than the glazed as it raises 45° F water to 85° F. Only the last 30°, bringing the water temperature to 115° F, are more difficult for the unglazed collector with a selective surface. In Albuquerque, over the course of a year, the unglazed selective surface is more effective than the glazed. Plain black non-selective collectors have a tougher time in winter, but still function.
The skeptic, who has no back-up water heating, is right to ignore my arguments and glaze his collector because it will outperform the unglazed collector during very cold, windy and hazy weather, the times when you really need it. Nevertheless, taking into consideration expense, having to protect glazed collectors against overheating and the shorter life of tanks full of very hot water, I believe the unglazed solar water heater to be a better choice for Albuquerque. This is true especially if you have a back-up water heater (I don't) that you can get a final boost from during cloudy weather.
I recommend buying an infra red thermometer that gives surface temperatures by merely pointing and clicking a button. Radio Shack sells a small IR thermometer for $30. With this invaluable tool, a walk through a parking lot is a lesson in the effect of colors and orientations on stagnation temperatures. At stagnation the temperature neither increases nor decreases, for the sun supplies heat to the surface at exactly the rate the surface loses heat to its surroundings. This happens quickly with the thin sheet-metal body of a car, and also the fur of a cat or a dog. I have found surface temperatures of black cars and cats 90° F above ambient. Green gets almost as hot as black. The stagnation temperature is the upper limit of what one can get from a collector.
Even in bright sun, white shaded surfaces facing the cold sky will drop considerably below air temperature. On clear nights the tops of cars of all colors can drop 20° F below air temperature. How are you to find air temperature? At what can you point your thermometer? Take a business card and wave it rapidly in the air somewhere out of the sun. This gives air temperature a chance to dominate the card's temperature rather than distant sources (or sinks) of radiation. You have to catch its temperature quickly. Of course, you could also drive 40 mph and hold the thermometer out the window, pointed at a shaded door panel. You would soon get close to ambient temperature.
Summer nights with an IR thermometer are just as exciting for the energy prospector as winter days. Just as you were delighted to find free heat in the sun, you will find free cool in surfaces pointed at the night sky — cool that you can gather and use on hot days.
None of this is new. Decades ago solar pioneers, including Harold Hay, Ray Bliss and Harry Thomason, built heating and cooling systems that exploited this. Somehow their work was ignored. Giant budgets and brilliant minds moved in other directions. Their work was rarely cost effective, but they always had numbers to prove they were right. Let us hope that today, when each of us can arm himself with a cheap Radio Shack handgun that spits out digital temperatures as fast as you can click, we can fight back and defend low budget, cost effective projects.
— Steve Baer, Zomeworks
Source: NMSEA SunPaper (March/April 2003). Text extracted by OCR from scanned document.
PDF: 2003-03-01_unglazed_solar_collectors.pdf