When comparing peak efficiency levels it may seem that there is little difference between flat plate and vacuum tubes, in fact flat plate may actually be higher, but this is during minimal heat loss conditions. When averaged over a year vacuum tube collector have a clear advantage. The key points are:
1). Due to the cylindrical shape of the vacuum tube, the solar tubes are able to passively track the sun throughout the day. Flat plate collector only provides peak energy output at midday when the sun is perpendicular to the collector's surface.
2). Air is evacuated from the solar tube to form a vacuum. This greatly reduces conductive and convective heat loss from the interior of the tube. As a result wind and cold temperatures have less effect on the efficiency of the evacuated tube collector.
3).Ontek solar collectors can often be used in subzero temperatures without the system sustaining damage. Flat plate systems often require expensive and complicated "antifreeze" systems to be installed.
4). Vacuum tubes are strong, long lasting, and should one be broken, inexpensive and easy to replace. If a flat plate collector panel is damaged the whole panel must be replaced.
5). Due to the high efficiency absorption of solar radiation even during overcast conditions, combined with excellent isolative properties of the solar tube, solar tube collectors can heat water all year round (backup from gas and electricity is still required).
6). Due to the various advantages of vacuum tube collector over flat plate collectors, a smaller collector can be used to provide the same heating performance. For example, a standard household of 4-5 people would usually require a 250-300L water storage tank. Depending on your location, only 30 vacuum tubes would be required to provide all summer hot water needs and a large percentage in other seasons.
7). Flat plate solar collectors can produce similar heat output to vacuum tube collectors, but generally only during hot, sunny conditions. When averaged over an entire year, vacuum tube collector heat output per m2 of absorber area, is between 25% and 40% greater that a flat plate collector.