Pros, Cons & Conventional Wisdom
Efficiency. Collectors operate most efficiently when the
temperature of the inlet fluid (Ti) is the same as or less than the ambient temperature (Ta) of the air. When Ti equals Ta, flat-plate collectors tend to be about 75% efficient, while evacuated tubes have an efficiency of about 50%. However, collectors rarely operate under these conditions.

In most systems, collectors operate 30°F to 80°F above
ambient temperatures to produce end-use temperatures from 100°F to 130°F. As the inlet temperature increases, the potential for heat transfer from the absorber to the surrounding air increases-heat lost to the atmosphere is heat not transferred to the fluid in the collector, and the result is less efficiency. Because of the superior insulation in ETs, their efficiency curve, which shows the loss in efficiency as the difference between inlet and ambient temperature (Ti-Ta) increases, is less steep compared to flat plates. Flat plates are more efficient when Ti equals Ta, but the efficiency curves of each, which decrease at different rates, intersect at some point. Past this junction, as Ti continues to rise, ETs are more efficient than their flat-plate counterparts.
When comparing ET and flat plates having similarquality absorber plate coatings, this intersection typically
occurs when the inlet temperature exceeds the ambient by 90 to 100°F or more-conditions that most systems do not typically experience. However, it does mean that ET collectors are capable of producing higher temperatures overall and can produce more heat in cold weather. ETs also perform much better under cloudy and windy conditions, again a result of the improved insulation keeping more heat "in the collector."
Unfortunately, the superb insulation that otherwise helps evacuated-tube collectors can undermine their efficiency in areas that receive a lot of snow or heavy frost. Light passing through frost or snow will heat the absorber plate of flat-plate collectors. Some of this heat radiates out and warms the glass, melting frost and creating a layer of water that allows snow to slide off. However, in ET collectors, the more effective vacuum insulation prevents the heated absorber from warming the surrounding glass, resulting in much longer "melt-off" times.
Additionally, snow can pass through the spaces between the tubes and accumulate underneath, resulting in snow buildup. In some cases, evacuated-tube collectors may take half the day or more to melt snow or frost, reducing their operational time and offsetting their increased cold-weather efficiency

Design Differences. Typically designed with an unsealed enclosure, flat-plate collectors can be prone to condensation buildup on the inside of the glass as they age. Cosmetically, this may be an issue, but the impact on performance is minimal-an increased possibility of corrosion on the collector materials and mounting components is the most serious longtermeffect.
On the other hand, sealing and maintaining a vacuum is difficult, and an evacuated tube without a vacuum performs
very poorly. This was a common problem that plagued early designs that relied on seals, but today the majority of ETs use a continuous piece of glass to minimize the risk of vacuum loss.
Flat-plate collectors tend to have stronger glass than ETs. If the glazing breaks, though, replacing it can be challenge.
Due to the modular design of ET collectors, individual tubes can easily be replaced if they become damaged.
Siting & Other Considerations. Evacuated tubes are less sensitive to sun angle and orientation than flat-plate
collectors-some tubes can even be individually rotated within the rack system to favor late or early sun. Their
circular design allows sunlight to pass at a right angle through the same thickness of glass throughout the day,
whereas the changing sun angle relative to a fixed flat-plate collector results in increased reflection due to the angle of incidence.
Because of aesthetics or other constraints, collectors are often mounted parallel to the roof surface. While a
steeper pitch may favor winter production, many roof angles are 35° or less, and a parallel-to-roof installation can lead to collectors overheating in the summertime and under-producing during the winter. Again, for the reasons
mentioned above, this has less of an impact on ET collectors than for flat plates.
In general, ET collectors are best suited for areas with low winter temperatures and/or a below-average solar resource, or when high-temperature water is required (such as in some commercial applications). Realistically, either type will work for most applications, with flat plates usually a more economical choice in Sunbelt climates. Your local solar pros should have a handle on what works best in your particular climate.