The
United States' (USA) Environment Protection Authority (EPA)
states that evacuated tube collectors are "more
efficient than flat plates for a couple of reasons. First,
they perform well in both direct and diffuse solar radiation.
This characteristic, combined with the fact that the vacuum
minimizes heat losses to the outdoors, makes these collectors
particularly useful in areas with cold, cloudy winters.
Second, because of the circular shape of the evacuated tube,
sunlight is perpendicular to the absorber for most of the
day."