Solar Water Heating--Revised & Expanded Edition. Bob Ramlow
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Collector Efficiency
No matter which manufacturers you talk to, they will probably claim that they offer the most efficient collector. The truth of the matter is that they are all wrong. There is no such thing as the “most efficient” collector. For instance, consider evacuated tube collectors. Since their conception, much has been said about their performance as they are commonly heralded around the solar industry as a more efficient collector. If you were to look just at the collector design, that’s an easy assumption to make. Normally, if you reduce heat loss, in this case by the vacuum of the tubes, you would increase efficiency. So it may sound counterintuitive if we tell you that pool collectors, which are simply plastic tubes placed out in the sun, are actually more efficient for some applications even though they are not as well insulated. It takes more than a clever design to produce hot water. It takes a system that is appropriately suited to what you want it to do.
The measure of a collector’s efficiency should really be how it performs when put to use. The best way to measure that is through an independent testing organization. One of the good things that came out of the late ’70s solar boom was the Solar Rating and Certification Corporation (SRCC). The SRCC rates and certifies many of the collectors on the market today. It is the most common and reliable source in the United States for independent information about solar collectors. The SRCC does not perform the required tests on the collectors. The test was developed by American Society of Heating, Refrigeration and Air-conditioning Engineers, and is performed at accredited testing facilities. The SRCC uses the test results when rating the collectors We strongly suggest buying collectors that they have certified. Not only does the test calculate collector performance and efficiency, it also tests for durability and reliability. Both are critical for determining the value of a collector. The results are free to the public and can easily be accessed online at solarrating.org. Using the SRCC gives us good, solid standardized data for comparing collector performance.
When rating a collector, the test measures the amount of heat, in Btu, that it will produce, based on a certain amount of radiation that shines on the collector. The testing facility usually does this with big lights to ensure consistency between tests, but some facilities conduct the tests outdoors using real sunlight. Because the solar resource is inconsistent, three conditions are considered: clear day (2,000 Btu/ft2/ day), mildly cloudy (1,500 Btu/ft2/day), cloudy day (1,000 Btu/ft2/day). The conditions mimic how the amount of sun will vary depending on location and climate. As a second variable, the test will alter the temperature at the site. This is actually the difference of the temperature of the fluid going into the collector (inlet temperature) and the temperature outside (ambient temperature). This gives you a measure of how hot the fluid is that you are trying to heat and how cold it is outside. Figure 3.9 graphs the ability of each type of collector to convert sunlight into usable Btu for all of the temperature variables. The data is an average of all three sun conditions and was taken from a sample of ten manufacturers of each type of collector to provide a measure of overall performance.
Figure 3.9: Mean collector efficiency ratings
As you can see, when there is very little difference between the inlet temperature and the ambient temperature, the pool collectors are significantly more efficient than both flat plate and evacuated tube collectors. Does this make them the most efficient collector? No. It simply means that they are better during some conditions. Similarly, the flat plate collectors are more efficient when the inlet/ambient temperature difference is between 10°F and about 70°F. After that point, the evacuated tube collectors become more efficient. The efficiency of the collector is entirely contingent on where and how it is being used.
The question now should be, where does my situation fit into this? For most domestic water and space heating applications we are trying to get our fluid up to 120°F–140°F. Let’s consider an example in which you have a system design to heat your domestic hot water. Let’s say it is 50°F outside and the fluid returning to your collector is 100°F. In this condition, you would look to the point at 50°F on the graph. Flat plate collectors are about 40 percent efficient, and evacuated tube collectors are around 34 percent. That’s quite a difference in performance, even on a relatively cold day. If you are properly dumping the heat, the inlet temperature on most residential applications is usually 100°–110°F at most. However, at that point you will not need much more to reach your desired temperature.
We have found that for most residential water and space heating conditions, flat plate collectors will outperform evacuated tubes. Now, if you needed really high temperatures, say higher than 160°F, then evacuated tubes might be the right collector for the job. Like we said before, it all depends on where and how it is being used. Pick the right tool for the job.
The second claim made for evacuated tube collectors is that they are better collectors during cloudy conditions. Figure 3.10 graphs the efficiency ratings for all three SRCC conditions, including cloudy, low-sun weather. As you can see, the point where the collectors’ efficiency ratings cross is less than the average, signaling an increased efficiency. However, they are still not more efficient than flat plate collectors in most temperatures.
Additionally, you need to consider the value of a system being better at harvesting a decreased resource. If there isn’t much solar radiation to gather in the first place, being slightly better doesn’t amount to a whole lot of Btu. More of a little bit is still only a little bit.
Durability
Although much of the industry debate has centered around efficiency, as it is the obvious selling point, we have tended to focus more on the long-term performance of collectors than on the short term. The system you install today should be designed to last at least 40 years. There is no substitute for quality. The collectors on Bob’s home were installed in the early 1980s and are still in great condition.
An easy indicator of the quality of the collector is the warranty. Most come with a 10-year warranty, although some have stretched this to 15 years. You might also want to check into the manufacturer. How long has the company been in business? A few companies have been making collectors since the ’70s and ’80s and have continued to put out long-lasting products. You may also want to find out where the manufacturer is from. We have several companies here in the Midwest that we like to deal with because they are local. This not only cuts down on shipping and transportation costs; it also allows us some flexibility if we need something in a hurry.
Figure 3.10: Collector efficiency ratings over all conditions
Climate and Location
In some climates, such as ours, snow is a significant factor in collector performance. Snow can accumulate on the collectors and diminish the solar resource. Flat plate collectors have a distinct advantage over other types in that they shed snow very well when installed in climates that experience significant snowfall. The large pane of glass on the front will lose heat, causing the snow to slough off. An issue with evacuated tube collectors is that they do not shed snow. Because the evacuated tubes are such good insulators, little heat escapes, and the snow that accumulates on the tubes can stick for a long time. Their surface is also irregular, so snow packs between the tubes as well. We have seen instances where roof-mounted evacuated tube collector arrays got packed with snow in the early winter and stayed that way till spring, which rendered them completely useless for a good portion of the year. The lesson here is to always mount evacuated tube collectors at a significant angle if used in a climate that experiences snow. Also, never flush mount evacuated tube collectors in a climate that experiences snow.