Flat plate solar water heaters are only at their maximum efficiency in the middle of the day. A great deal of the sun’s energy is simply reflected off the flat glass panels, especially in the morning and afternoon.
Because evacuated tubes are cylindrical, they are always facing the sun and therefore they can absorb more of the sun’s energy.
The manufacturers claim the evacuated tube collectors will absorb up to 40% more of the sun's energy that a comparably sized flat plate solar collector. The chart below clearly demonstrates the improved efficiency of the tubes, particularly mornings and afternoons where the sun will not be directly above the panels.The tubes consist of 2 layers of toughened glass with a vacuum between these 2 layers. This evacuated layer in the tubes acts much like a thermos flask, retaining about 97% of the energy absorbed from the sun. This helps increase the efficiency of the collector and also protects it from the effects of cold air (the water in flat plate collectors can freeze in very cold areas). This makes evacuated tube solar collector the perfect system for very cold areas.
Check them out at Endless Solar