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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.