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.
Absorber coatings will influence the performance of a solar thermal collector, and the amount of this effect may be more critical depending on the climate the collector will be placed in. Some types of absorber coatings have a higher performance rating than others. In climates that experience a high number of sunny days per year and also experience relatively consistent warm temperatures, collectors can perform well with less efficient absorber coatings, whereas in climates with fewer sunny days per year and colder temperatures, collector coatings with higher efficiencies work best.
You may also need to consider the density of the absorber area compared to the overall footprint of the collector. Manufacturers and certification agencies use the terms gross collector area, net aperture area and absorber area. Gross collector is the entire area including the frame; aperture area is the size of the glass; and absorber area is the amount of surface that will actually absorb solar radiation. In flat plate collectors virtually the whole collector area equals the absorber area, with only about an inch around the edge being frame and not absorber. In an evacuated tube collector, there is space between each tube that is not absorber area. In most instances a flat plate collector will take more than 25 percent less space than an evacuated tube collector that has an equivalent absorber area. This may be an important factor because a common limiting factor in siting collectors is the amount of available mounting space. In other words, you want to have enough space for the amount of collector area you want, so to get as much heat as you can, you want to have a type of collector that has the best gross-to-net absorber ratio.
Note that the amount of solar energy that falls on a square foot of the earth is a constant and cannot be changed by the type of collector used. The primary way to increase the amount of energy collected is to increase the absorber area.
By now it has probably become clear that we are not completely impartial when it comes to collector selection. It is difficult to remain unbiased while still trying to provide the knowledge we have gathered through years of experience installing, maintaining and designing these systems. We have seen the best performance from flat plate collectors, and we want you to have the same success. We have no doubt that evacuated tube collectors have a secure place in the solar thermal industry, especially in high-temperature applications. For instance, the emerging solar thermal-powered air conditioning systems that use evacuated tube collectors to drive single-or double-effect chillers hold great promise. However, for the majority of domestic water and space heating applications, flat plate collectors have a proven track record.
Air Collectors
Up to this point, all the kinds of collectors we have talked about have used a liquid as the heat transfer medium. Air can also be used as the heat transfer mechanism in a solar collector. Air collectors are flat plate collectors and share all the same characteristics of liquid-type flat plate collectors in size and construction. Instead of an absorber plate made of copper piping and copper fins, the absorber plate in an air collector is typically made of a solid sheet of aluminum. The aluminum absorber plate is coated with a selective surface or black paint and is usually dimpled to increase efficiency. When the sun shines on the absorber plate, it gets hot. Air is drawn
